I want to use up some chocolate I have on hand, and don't have enough of either one for the recipe I plan to use for truffles, so I thought I'd try combining the 3 chocolates. One is 54%, one is 63%, and one is 86%. Any suggestions or warnings would be appreciated! Thanks!
You're first melting it all and mixing it together, right? You can just compute the cacao contents of the resulting mixture. For that you'll need to weigh the three batches. Say the 54% batch is 200g, the 63% batch is 150g and the 86% batch is 100g. Then you have a total of 54% * 200g + 63% * 150g + 86% * 100g = 108g + 94.5g + 86g = 288.5g of cacao out of a total mass of 200g + 150g + 100g = 450g. So the resulting cacao percentage is 288.5 / 450 = 64%. Now just treat the mixture as you would 64% cacao chocolate (since that's what it is).
Mixing chocolates for tempering is usually fine. The percentage of cocoa isn't actually what makes a difference in the tempering. The percentage of cocoa butter is what really matters. Unfortunately, that information is usually hard to find, so most tempering techniques just assume an average amount.
As long as all three chocolates have been properly stored (cool, dry, sealed), then you should have no problem. If you are using the seeding method, save the best stored or newest chocolate to use as the seed, since it will be most in temper.