Let me explain about the way the signs work when they leave their domain, in my opinion, to give their rulings over to another province or house/signs ruler-ship. A sign has natural rule over the house he naturally has significance over or tenants, (Aries 1st, Taurus 2nd etc.) Sagittarius as a sign/house or kingdom rules the 9th house which is significant of long journeys and foreign soil concerns. When the 9th sign or kingdom of Sag. sends a ruling or lord of his sign to another house or province he in effect either lending his sovereignty of/to that house's concerns, or he has invite under the guidance of another estate to aid or otherwise on the original estates behalf. (a Queen or Kings concerns-fixed stars)
When lets say Sag. (the way I normally (Sag.) think about long journeys) now comes to the (third house) a different approach of 'thinking processes or expectations' of different values will be given ref. to with of a third house type of thinking. (third house rules common thinking processes which-every day common place activities give significance to.) You will then take into consideration the relationship the Lord of the third has with Jupiter and Saturn, in the case here Mercury trines Jupiter and sextiles Saturn, they are all in agreement to their realms concerns. Normally Gemini and Sagittarius are opposing signs but the topics of their concerns are in unison of agreement now.
Yes Sag. rules long journey's but is now come to visit and colour a different meaning of house tenancy and now has a different approach of the way third house ways are dealt with or thought of, here in a common expression of expectation.
Additionally Jupiter-Sag. ambassador is deposited within your first house also saying, I have given my venture over to the rule of the sign of the place I send my ambassador, in the case above it is your personal first. The affairs of the journey are being planned by you physically and you have first hand of its initiation-you will be making the journey, it is your decision is it not?
Jupiter makes a mutual reception with Saturn in the third so Jupiter is able to get out of the via combust burning way of fixed stars it is known for, which would normally make Jupiter's significance there mildly ineffectual. (Jupiter is the great benific so for his presence to be solely ineffectual, it is my belief he would also have to suffer some other ill dignity.) But..he is now able to see himself in dignity in the third and forth domains, where his kings reference by sign (house) has significance in meaning in Sag. (third), Jupiter/Saturn=resonanating both in 3rd/4th h.'s tenancy, some influence in the sign as a shared vibration in last decision making (4th, Capricorn, Saturn is lord of,) or end efforts of the journey or the activities he has best planning or representation of?
Hoping I am making the explanation understandable of just how the energies may make play to their efficacy.
When the kingdom or all the kings horses and all the kings men, solar system planets representing realms or estates of the fixed stars the kings lower powers of representations-metaphorically speaking, are sent.. (His kingship meanings as lets say Regulus as a fixed star is a much better representative of a realm or kings domain..Regulus is a most beneficial prominent star in the heavens-ruling riches - prestige etc.. (A planet would possibly not be able to give the same exact meaningfulness in weight or tenure of value as that Fixed star does.) The fixed stars do not move about the signs as the solar system stars or planets do therefore in a scheme of men's affairs they are more powerful to their kingdoms reason of being=events activities which the planets reference or the solar system planets do.) they give a special value and power or colour, a different cloak you might say of appearance of any subject matter. The signs give kindred applications to the planets way by use of the original signs/house meaning or tool of their designs intent.
We often think of the Solar System planets as rulers or lords representing the signs/houses but it is these fixed estates or stars which are best delegates or representations of the kings purpose of the sign/house or estates power-they have significance in the scheme of men's destiny mundanely speaking of course, yet they show the temperament and different hue the affairs of men should be directed of by origin of the signs they represent, they are lower closer to us, but it is the higher value we must always keep our sights to, the originators, the fixed stars give reference to and are of long standing of. They represent stages/houses which never change in meaning, they give intention's of the signs meanings. They are not per say ambassadors as in the likeness of planets representing the kingdoms sent as delegates to other provinces by invitation to help negate or give power of command by. They direct their decisions in the event at hand in the question topically or by transit. They give reference to the next energy 'under' the meaning of the different sign or estate they find themselves in according to the seasons of men.
There fore as Satrun is in long journeying Sag. on your third, you will give some serious thinking to the every day thinking (third house) topics of long journeying.
Another view: Saturn is in Sag., the sign/house does not naturally exalt his presence there, but he is a most honored guest back in Libra in mutual reception with Jupiter. In Libra he has weight to the activities where Libra gives significance honor, rule. Venus is a lord or lady of Libra and Saturn's presence there makes her significance more dignified as a resonance there. He in Libra is legal, a standing under the law of heaven's grace, metaphorically speaking, all things needing balance are glorified and set in proper standing with Saturn in Libra in accordance with the query/natal subject matter of the chart..the season of Winter is about to beset a decision.
Hope I have made things a little more clear?
All the best
Vyri