5 minutes does not seem long. It is enough though to get major results. When you pick up a weight to start a set is it very tough at the beginning? No, not if you are planning on lifting it for 12 reps.
Earlier reps are just to wear the muscle until you get to the last and most important reps. The last 2 are the key failure reps. You want to do reps to failure.
If a given set consists of 12 reps, then reps 1-10 are only performed in order to get to reps 11 and 12. Reps 1-10 will do very little in terms of stimulating muscular growth, but are necessary to perform in order to overload the muscles on reps 11 and 12.
In other words, it is only the last 1-2 reps that will ultimately yield a muscle building response from the body.
If you are doing 12 reps, a minute and a half is all you need to build to failure. Then move to your next exercise.
Your success is literally measured by the brief moments in time at the end of each set and the amount of effort you are willing to exert during this time.
Losing a few pounds can really help with the looks or definition of muscle. And muscle mass helps with that. Muscle helps burn fat. So this becomes a glory loop.
So how much can you work out? At 3 workouts per 5 minutes, how many 5 minutes can you squeeze in in a week? Should be many. One thing I like about a 5 minute "muscle" break is I usually do not even start to sweat so I can do it without showering.
When you can hardly move the weight is when you need to dig deep. It is the last rep that counts. You can do it.
The end of the set is super hard but focus helps you get through. And you can train yourself to focus. And remember - you can do almost anything for 5 minutes.
One trick I like to use is to use a timer. Set the time to 90 seconds per set - then do it.