well, thanks for all the advice! it is amazing how much help we get from you! Thank you all very much!!!
Since we have ordered the plywood already, I think we will use it and hope it works fine. it is hardwood, so hopefully... We are laying a small area in the shed to test it before we put it in in the whole house... so if there is a major problem we should find out after a few weeks of testing... The question is now: Silicon or flexible tile adhesive in the 3mm gaps? would the flexible adhesive also be flexible enough for the expansion of the wood? what are the disadvantages of Silicon? i conclude from what all of you wrote, that both should possibly work fine... any "objections" .
Thanks again!!!!
Alexander.