As a self proclaimed tool-guy I often get asked about free 3 way merge/diff tools. Most people use trial/cracked Beyond Compare or Araxis Merge.
I can’t remember when I last needed 3-way merge, 2-way serves me just find and I use WinMerge and TortoiseMerge all the time.
Anyway, these are my 3 suggestions for free 3-way/merge tools…
portable | 32/64 bit | free + open source | windows, mac, linux
portable | 32 bit | free | windows, mac, linux
Perforce Visual Merge Tool