Strange enough I have experience with all of these 3 bikes in a way. So let me start;
XT660, the new version is monocylinder. I used to have the earlier XT600E. I can see the improvements, more responsive and stronger engine, decent headlight and dash (mine did not have the tachimeter), some wind protection. Probabily the most dirt inclined of the bunch. Only drawback the exhaust pipes are routed below the engine. Easy to hit. Probabily you can fix that with decent engine guard.
If you ride solo, don't mind slower speed on asphalt, and some vibrations I would have taken this one. With 175kg is lightest bike in the group.
Transalp, has been around for years. Bullet proof engine. Very few quirks. I had a 600cc version from 98. Very impressive bike. More roomy than XT better wind protection, twin V means less vibrations. Give it a good set of tires and same bike will take you over the mountain paths and highway at the same ease.
Heavier that the XT660 (191kg), lacks power on higway compared to DL.
DL650, now things start getting a bit exciting. After selling my TA I have purchased the DL1000. Reasons were, travelling 2 people I needed more space better performance and comfort. I did manage to go over dirt roads and gravel paths with DL1000. I have also tested the DL650 on a short ride.
Advantages of DL650: Powerfull engine compared the TA. Lot of space for 2 person to travel, solid engine.
On the other hand the front tire is 19" and others are 21". The bigger tire is an advantage at offroad. More Heavier that the X660E, (194kg)more or less same weight with TA.
I would have focused what type of riding I was going to do before choosing one. If it is a mix I would have gone DL. Add engine guard, crash bars, hand guards, chain oiler, side cases, cross bar at the steering, lighter outlet, some grip heaters.
If you are going to be prefering lesser traveled roads than XT660 would be the part.
On the other hand if you run into a cheap second TA even if it is pre 2000 (600cc) you may give it a try. The saved money can be spend to accessories and to trip.
Best of lucks
EMOK :: Enduro Motosiklet Kulübü