Both of the rides you want are Tier 1, and you can book only one Tier 1 attraction ahead of time. So pick the one you want the most. The other Tier 1 attraction is Soarin', which is probably the only other ride one might need an LL for.
The Tier 2 attractions at Epcot are rides that rarely need the LL, but might as well book your 3 in advance, in which case I'd make SE and maybe Nemo as your other two.
The trick with Epcot is to rope-drop (assuming you're staying on-site and have Early Theme Park Entry) and do one of the Tier 1s then. But if you want to do all 3, you're going to end up waiting in at least one line.
ETA: Posted before
@scrappinginontario did! Her post is actually more informative...and shorter!