Woggy, I've been in positions where my team has been rushed by 3 flame tanks and we have no weapons factory to create tanks. Usually in that type of situation, you'll need total cooperation by the team. A few people will need to be hotwires and have a few be gunners. Gunner does a fairly good job chipping away at a flame tanks armor and most of the time the guy operating the flame tank will keep trying to take down the same building while your guys are keeping it repaired.Now if your barracks is destroyed as well, this would be more difficult because you'd have to used grenadiers instead (which can be useful if you are skilled in aiming them).
Otherwise, you could take your chances laying c4 on them, but it's pretty hard with a ft considering their mobile ability.