Okay. Here I go. My first ever big long speech:
Let's go
1.Tidus: To get his ultimate weapon you can find it in the north west of the Calm Lands. Someone should be standing in the way but to move him, win a Catcher Chocobo race. To get the Sun Crest, go down the stairs in the North of the Zanarkand Dome - The Beyond to another room where you can get it. To get the Sun Sigil, complete the Catcher Chocobo race in under 0 seconds
2.Lulu: The weapon is behind some rocks in the Ruins - Underwater Hall, the place where you met that boss sort of at the beginning of the whole game. To get the Venus Crest, go to Guadosalam - The Farplane. It should be there. The Venus Sigil : dodge 200 lightning strikes in a row in the Thunder Plains. Go to the front of the Travel Agency to get it after.
3.Kimahri:Press square in front of all the Qactuar stones to get a ghost that lead you to Kimahri's weapon. To get the Saturn Crest, start from Gagazet - Mountain Cave, go back through Fayth Cluster and into Prominence. Should be somewhere there. The Saturn Sigil is in the Macalania Woods after beating both the butterfly hunts. That's North and Central. Well, there you go call if you need anything else!