I have developed many riffs but do not know how to develop them into ful blown 3 minute songs

try using the one riff for the verse and maybe changing around the same chords for the chorus

Or grab a few of your favorite home-made riffs, then throw them together. One could be the verse, one could be the prechorus, one could be the chorus, one could be the bridge etc.
:D

Best thing I can think of suggesting is to experiment, experiment alot. If you haven't already recorded all of you favorite lines I'd suggest doing so. Pick up a cheap four track cassette recorder and lay all of you stuff on it. Then you can sit back and listen to what you have without being caught up in the moment playing it. You may be surprised at some of the ideas that'll pop into your head while your away from your instrument and just listening.

Eventually you'll come across pieces that sound like they would work well together in a song, experiment with arranging them in different ways.

The more time you put into arranging your bass lines, mixing and matching different pieces, the sooner you'll begin to hear songs evolving.

That's basically how I go about writting for the most part, hope that helps, good luck.