Suddenly everyone agreed, having caught cold (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this in another clue)
'everyone agreed having caught cold' is the wordplay.
'everyone' becomes 'all' (all people).
'agreed' becomes 'at one' (to be 'at one' is to be agreed).
'having caught' indicates putting letters inside.
'cold' becomes 'C' (eg on taps).
'atone' going around 'c' is 'atonce'.
'all'+'atonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "in unison" , "Together" , "Suddenly" , "Simultaneously - immediately" , "Without warning" .)