You really don't need to defend all that much in this spot.

TOTAL defense across all three of you has to be 62.5%. So on average each person has to defend about 21%.

I think it's safe to assume that the first caller (even though they are a weak player) might have the strongest range, so we can assume they should cover a bit more of the defending in this spot. Not to mention the other two ppl are IP and would have an easier time defending.

So maybe the first player defends 30%, second player 20%, this really means you only have to defend about 12.5% of the time. You shouldn't have too hard of a time doing that. Being the last caller you probably have a fair bit of Kxs and stuff.

Morale of the story: Your need to worry about defending frequencies drops considerably when the overall defense can be split across multiple ppl.