What I would do is consider selling the 60D with your lenses and talking to a local store about bundling a 5D mark II with a 24-70 and pick up a used 550D (same sensor as the 60D) for a back-up. A 5D body with a used lens is something to consider to save.
I'm not sure the turnaround on that conversion is really worth the, what, $200 you'll get from it. The 550D body feels like crap in your hand; which is a big deal if you'll be holding it for several hours at a time. Then we can talk about battery life (nearly 3x as long with the 60D and the compatibility of 60D batteries with the 5D and 7D), how easy the controls are to work (much easier on a 60D, though not as good as the 5D/7D); not to mention AF and burst rate. There are other smaller things; but the ones listed above are reason enough not to downgrade.
I'd sell the 18-135 ($200), unless you see yourself using it for travel, etc. Then get yourself a 5DmarkII to complement your 60D ($1500 through Canon Loyalty Program)...then I'd consider where you are shooting those 3 weddings. If they are outdoor weddings, I'd get the 70-200 f/4 IS and mount on the 60D($1000ish used), and a 24-70 f/2.8 ($1000). If they are indoor weddings, then you want to go more primes for low-light handling. Either way, you should have leftover money for another flash, even if it's another 430ex.
Lots of ways you could go...but, Id say going the full-frame low-light handling route would be the most ideal to complement the 60D. And going 5DII over 5DIII allows you a lot more room for new glass. Then maybe next wedding season you'll upgrade the 60D to something else