If you send via click and send then you can combine the 3 items into the once parcel and the tracking number is the same for all 3. I combine postage but some people obviously aren't fussed and just pay full postage on all three. Touch wood, but I haven't had a problem with sending them in one package together.

Check they are all the same Paypal address (that's the important one) and if they are, I think it's ok to send them in one lot. Just make sure you have tracking and proof of lodgement to cover yourself!

Quite a few sellers won't refund the difference in combined postage because it does muck up the payment status on the Ebay site, if you clearly state you do combined postage and they don't ask for it, then I think you've gone above and beyond by contacting them.