To answer the original question...
Check out these lessons, from Fareed Haque:
http://www.fareed.com/lessons.html
Jazz Theory Made Simple and Jazzy Sounds for Rockers should get you started on how to start soloing. Fareed's main points are to learn arpeggios for each chord form you want to solo over, and then mix these into the soloing you are already doing.
So, you ask about a G-C-D progression. Well, that looks like a blues I-IV-V to me. Some things you could try would be a G major scale or a G blues scale (minor pentatonic). But, that can get boring. Mix in riffs around the G7(9, 11, #11, 13) over the G chord, and so on for the other chords. When using arpeggios, just land on a note in the arpeggio, you can come into it from a note above or below it. Also, any note sounds cooler if you slide into it
See if those free lessons don't help you, Fareed's a great teacher.