I was 26/27 weeks when we went to Greece/malta a couple summers ago. I think you can travel without a certificate up to 28 weeks, after that some airlines might ask for a fit to fly certificate but cut off depends on an airline, about 32/36 weeks
I went to St Lucia at 20 weeks only thing I had to change was my scan date! But flying was fine just had compression socks and kept walking up and down the plane x
I mean this is only my personal preference, but I would stop travelling abroad in my 3rd trimester and just stick to places closer to home just in case.