How to maximize prestiging while having most time efficiency?

For each PKM you defeat, prestige gain is based on your highest CP on the team (even if not used in battle).

[Edit - deleted example - see other reply below]

Another alternative - just hit the first one and run - I recently hit a level 7 gym with 1400 Ursa, followed by 2 Blissey's - I actually had 2 Machop's under 700 (awaiting evolution) with OK fighting moves that were able to take care of him - required 8 rounds of fighting and revives/potions, but I was rewarded by putting in a 3rd blissey that has made that gym crazy strong for the area it is in!

I personally would pick my mon around 1900 CP in this case to go for an easy battle that would award 1300-1400 prestige total, rather than trying to get the most prestige in a single battle but also struggling to beat the defenders.

I would go for under 1000. This way the first mon gives you 500 (if it was 1001 you used, you'd be down to 255 prestige). The second mon would give you 1000 prestige. I'd run at that point. So, 1500 prestige per attempt.

Using a cap of cp1500, you'd get 153 prestige from the first, 667 from the second, and 1000 from the third. Total of 1820 prestige.

This also depends on what mons you have at what levels and who you are fighting. A 3000 blissey would definitely be ignored and I'd just face the first two. A 3000 Rhydon would seem my 14xx victreebel or tangela or similar.

I know, not your question, but i would concider not to be time efficient but potion-efficient.
Thus, i would just use those single-use-pkm (ratticate, pidgeotto,....) in 800-1000 range and prestige the fist both defenders and transfer them after faint.