Assuming AC won't interline your bags over to AF or BA (they should):

You can easily walk from T3 to T2 in 10 minutes or less.

T3 to T4 is more difficult. You have to take the Heahrow Express to T4 (it's free). Walk to the station is just as far as to T2 and then you have up to a 15 minute wait (service interval) and about 2-3 minutes transit time, + 5 minute walk to the departure hall at the other side (can be very busy too as it's centralised checkin for all BA flights).

You might want to reclaim your bags at LHR and recheck with AF or BA to minimise risk of loss. The only worry of course is if your AC flight is late.