Conditional Formatting - find in a list and if

[JLH]

I need to conditionally format column U which contains a % value.

Column P and column R contain values which can appear in only one of three lists: hi, med, lo.

What I would like to achieve is (using row 3 as an example but to work on the entire column):
If U3 is blank then no format

If R3 appears in hi then colour U3 red

If R3 appears in med and if P3 appears in hi, and U3<75% then colour U3 red
If R3 appears in med and if P3 appears in hi, and U3>=75% then colour U3 yellow

If R3 appears in lo and if P3 appears in hi, and U3<75% then colour U3 red
If R3 appears in lo and if P3 appears in hi, and U3>=75% then colour U3 yellow

If R3 appears in lo and if P3 appears in med, and U3<75% then colour U3 yellow
If R3 appears in lo and if P3 appears in med, and U3>=75% then colour U3 green

If R3 appears in lo and if P3 appears in lo, then colour U3 green

It sounded simple till I came to write it down. Grateful for any help
- cheers!