I would have thought that they would be monitoring the situation and ensuring avast is up to speed. Since much of SP3 will be a collation of all security updates after SP2 (in much the same way as previous SPs), there isn't that much new that is in SP3. Some of that is what is already in Vista so I think it won't be a huge task for Alwil.
Remember avast was one of the first AVs available for Vista in beta and RC forms and that would have been a much greater undertaking.
Obviously only Alwil can say what they are doing relating to XP SP3.