Any question answered...

I'd like to buy a portable HDD for pictures and films so I can plug it in the xbox and watch them.

Can I buy a USB 3 which will be compatible with USB 2 also? As in I can make the most of the USB3 speeds on my PC but it will read usb 2.0 on the xbox and macbook.

Would I be better off buying a portable HDD or buying a HDD enclosure and HDD?

USB 1,2,3 are all backwards compatible, buy a portable if this is your aim.
They are pretty cheap now, £62 for a 1TB from currys even.
If you want to use it on all 3 devices you are stuck using FAT32 so no 1080 rips because of file size.