I have thousands of kms under my belt on a KLR 650 - it is the only one of the three I can personally comment on.
I did a trip into Africa with my cousin some years back - me on the KLR and him on the XT660.
Neither of us had problems on the 28 000-odd km trip over eight months. We had hundreds of "little races" along the way, and the 660 would win most of the time (I think I had more luggage on the KLR...

). However, we ran into some diplomatic issues in Kenya and had to leave the country in a hurry - we had one night to strip our bikes down and pack them into crates to be flown out to Malawi, and I had a much easier time disassembling the KLR than my cuzzie's XT.
We also swopped bikes from time to time, and I must say that the 660 was more fun to ride than my KLR. That said, however, one can fix anything on a KLR with bloudraad and prestik. The KLR also survived a lot of harsher treatment than the XT, mostly because I was a poorer rider. The worst abuse it took was a vertical 3 meter drop onto its head... Picked it up, bent back the cock-eyed pieces, started it up and kept going.
I did find that for Africa, simpler is better - we did run into some guys on big Beemers (not the 650 Dakkar), who had run into some trouble, and were fairly stuck, waiting to be bailed from South Africa. There is, however a lot of BMW 650 Dakkar parts and support in Africa.
My advice would be to ride all three of the bikes on your list - you really can't go wrong with any of them.
Many happy miles!!