If you already have an 08 pickup, presumably you are looking for new or near new.
All 3, even the PowderSmoke are pretty reliable in their latest generation, either brands 6.7 or the LML Dmax.
If you're worried about steep repair costs, the probability of major engine issues in any of them in the first 100-150k mi is minor, however if you do have issues, open the hoods on all 3 and tell me which one you'd rather work on.
The PS you can't even see the engine. The DMax is still a crowded mess. The Cummins however, as well as being the longest running engine design, is relatively accessible and you can see everything once you pull that big stupid plastic cover off the top. I did my injectors in my 5.9 in less than 8 hours in my garage with basic hand tools. First time injector change for me too. Nota chance of that taking less than twice the time and about 25% more $ on the other 2. Plus the Cummins is the only CP3 injection pump powered truck I believe. The other 2 are CP4 powered and have had some problems. Nothing to steer clear of IMO but not as bulletproof as the ole CP3.
To top it off Ram arguably makes the best truck around its engine too. It's a win win compared to only a few years ago.