Yes so the real question is if Roddick could or would manage 3 Wimbledon and U .S Opens today; since he is likely not winning the AO, and of course not in a trillion years winning RG. It is possible perhaps, even 4 is not impossible, but I don't think it is a safe bet by any means he wins 3 either.
Some people use the "Roddick would win 4 or 5 slams without Federer" which is probably true, as an argument to convince themselves he is a 4 or 5 slam winning player in any other era. That really does not work. It is the same as Federer and Djokovic both probably win about 5 RG titles without Djokovic but I would be willing to bet money there is still no era you transport Federer and Djokovic into that either wins 5 RG titles.
I think an even better hypothetical is to transport both to an entirely different time altogether. In all honesty I think there is probably a higher likelihood of Wawrinka somehow getting to 3 majors even in the 90s (not saying he does, it is all relative to Roddick) when you think that when playing well he is clearly better than Kafelnikov who got 2 majors, and has similar surface preferences to Wawrinka. Higher odds IMO than there is of Roddick getting 3 majors in the 90s which would be very hard for him on those faster courts, especialy with his return of serve. Not saying it would be easy for Wawrinka to get 3 majors in the 90s via Australian Open and Roland Garros, maybe a lucky U.S Open somewhere similar to today. Or that Roddick couldn't possibly somehow get 3 Wimbledon and U.S Opens in the 90s. Just that if forced to bet my life on one or the other I would pick Stan to succeed in that before Roddick. I don't think it needs to be explained Roddick would have a very hard time overcoming prime Sampras at Wimbledon/U.S Open, and even Agassi who is a terrible match up for him when Agassi is playing well. Not to mention the obstacles and challenges from Becker (still playing well up until 96), Rafter, Ivanisevic, Krajicek, Edberg in the early 90s, Courier, Chang, and some others. Of course Wawrinka would have his numerous obstacles and roadblocks at his possible slams too.