Retro plain squares under the table, perhaps? (2,3,5)
I believe the answer is:
on all fours
'under the table perhaps?' is the definition.
(someone under a table might be on all fours)
'retro plain squares' is the wordplay.
'retro' says the letters should be written in reverse (retro is Latin for 'backwards').
'plain' becomes 'llano' (a grassy plain).
'squares' becomes 'fours' (four is a square number).
'llano' backwards is 'onall'.
'onall'+'fours'='ON ALL FOURS'
(Other definitions for on all fours that I've seen before include "in a defensive position" , "Maybe crawling" , "using arms and legs" .)