My vet will normally bring vaccinations forward a bit too; I'm not sure exactly by how much so I don't know if it's too close to their last VHD jabs - hopefully somebody more knowledgeable will know or you can check with the vet before you book.
If it is possible, then you could have the combined for all of them in August - including bringing Pickle's forward a little if you want to take all the bunnies at once.
Alternatively, I would go for the myxi jab around now for all of them (so bringing it forward slightly) & then, you could go for the combined jabs in December. Otherwise, if you do the myxi in August and then wait till Feb next year to do the combined, the trio's VHD will be a couple of mths out of date.