Alternatively, use the "Get Peak Level' feature of the Normalize UI on each of the three WAVs... each run will probably return a different value. Take the closest to 0 value and then use the "add gain" feature to add that much gain to each wav.
Ex:
Detect peak returns -6, -5, and -8 for each of the three wavs, respectively.
Then Add 5db to each WAV (although technically,I guess you'd only want to add 4.9 db so as not to hit 0 on the 2nd wav in this example).
Since the wavs are split up into reasonable sizes you will be less likely to encounter any 2 gig limit issues. Although it sounds like you still have 2 2-gig files which means you'll most likely need to split those files up further, or use smaller bit-depth temp files for the operations.