Is a person with Gemini Sun/Scorpio Moon/Aries Rising most likely to have an unstable personality?

You are right as some signs are more stable than others. However something like personality disorders and mental illness can affect all signs. environment does play a hand here. But you being unstable, i say no. A Gemini sun makes you an intellectual as you are an air sign. but Geminis do tend to bore easily so keep a cadre of friends that have many interests and are they themselves interesting. A gemini unfortunately can have quick mood swings as the brain is so active at all times that you can flip-flop easily back and forth with this sign. it is hard to remain focused on one thing for long. Again, make sure you have several interests and then you can use that brain for a purpose instead of a ping-pong game. (said only in the kindest way) You may have insomnia with this Gemini sun as you just cannot turn your brain off! The hardest thing i have when dealing with a Gemini is i never know if I'm going to get the happy gemini or the angry gemini. Try to find balance but don't worry if it's just not there. it's not supposed to be. You are a Gemini. With your Scorpio moon you have great depth. which is good because it helps balance that Gemini sun which is not filled with depth nor is is supposed to be. Scorpio doesn't trust easily and is secretive until they truly believe one is trustworthy. Two of my Scorpio friends took about two years to trust me! Use that moon to observe others as that is what Scorpio does best; You are a great judge of ones character. This is very helpful for your Gemini sun which tends to accept others without much thought (if they are interesting enough that is) Be grateful that you have this great ability to know others character. That is a wonderful gift to be able to separate the good from the bad. Be watchful when having difficulties with others because Scorpio has a gift to see others weaknesses and often this ability will be used when in an angry situation. Think before you speak so you don't hurt others feelings. You are spot-on when it comes to pin-pointing others weaknesses. So use that ability wisely. And last but not least your Aries rising. The 1st house is physical body, personality, and ones first impression. You probably are great when meeting people. Aries are generally out-going and active. They require action. You might pick up a sport or activity to use some of the energy proficiently. There is a tendency for Aries rising to be a little hot-headed and somewhat arrogant. Aries likes to be right. And they believe that they are right even when they aren't. Aways give others that you meet a chance to open up as Aries can be focused on self. That's not bad, that is just what they do as Aries is ruled by mars. Be watchful of head injuries and always go to the doctor if high fevers are present as Aries rules the head. Have any head injury checked out thoroughly. Hopefully there won't ever be a head injury but forewarned is forearmed. You have a great balance of elements with these positions. Gemini Air, Scorpio water, and Aries fire. That is a great all-around balance. Good luck to you and hope this has been helpful Bless you