I'm trying to get in touch with the previous homeowner, but is it normal to have 3 returns but only 2 of them working?
There is only one return line coming off of the pump into the ground, so the lines are either plumbed in series or the tee's are inground?
Good flow out of the two that are working and absolutely nothing out of the third.
Would they have installed 3 when the pool was built, but then only used 2? The 2 that are working are both towards the shallow end while the unworking third is towards the deep.