As long as they are still owners, they can stay on the EC but your problem will be if they don’t tune up for meetings (as it sounds like they won’t) and have to give their proxy votes to someone else.
The EC has to approve proxies before they can vote and that means there has to be a quorum at the start of every meeting and that means all four of the remaining members have to be there to form the quorum (the non-attending members’ can’t be part of the quorum because their proxies haven’t been approved yet).
This sounds like a very unsatisfactory arrangement and you should try to persuade at least one of them to resign so you can co-opt another member on to the EC to give you some wriggle room.
You will need to find new office-bearers from the remaining members although opne person can take on more than one role. Failing that,your strata manager can take on any or all of those roles – but it will cost you.
As a last resort, if none of the three departing members want to resign, you could hold a Extraordinary General Meeting to declare the three seats vacant and elect new members.