Should I do say 30 push-ups in a row (maybe 3 sets with 30 sec pause in between) or should I do 4-5 sets of 10 spread across the day when I want strength in my arm?
My main focus is tennis and swimming, not to build muscle!
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Whether your goal is to gain muscle, strength, or endurance, it is always better to do your exercises together (with short breaks between sets) than spaced throughout the day.
In general, 3 sets of one exercise won't actually give you any noticeable changes in strength or endurance. You need to do more. You can try adding some exercises that would help your swimming stroke such as the Plank Row, Kettle Bell Swing, and Medicine Ball Pushup, too. Alternatively, This Site also has quite a few good exercises for arms, directed more towards swimming - most of which would be transferable to tennis as well. A good 4-5 exercises per workout for 3-4 sets would be ideal.
But in your case, your best bet is to also do more swimming. If your goal is ultimately strength in your arms for swimming, then just do a lot of it! Swimming is a great workout for your entire body, including your arms.