If you have any decent photos of them/family pics/their house/pets/car etc - basically anything they really like, then getting a Christmas decoration with the photo printed on it is nice (most photo services like snapfish, Photobox, Cewe, Boots etc do nice photos on metal or china and some other things of you search around. Sort of thing you can give one to each person. Ditto if any of them wear glasses, a photo printed glasses cleaning cloth is cheap but thoughtful and useful.
Think around if there are any other of the odd photo printed gifts that might be enjoyed - personalised game of UNO, coasters, socks, etc. If any of you are any good at turning photos into cartoons or artworks they could also work well.
Cewe /Boots do photo advent calendars - fill one with pics of them, a few of you, things that you do together and send it to them so it’s there for them to open on 1st December , along with a nice note to say that you’re looking forward to seeing them at Christmas and thought they might like a little something to help them count down to the big day. They do them with and without chocolate in so you wouldn’t even need to spend on the choc version if there would be 3 of them opening it together. But it’s thoughtful and in advance when they aren’t expecting something and they don’t need to do the same for you, but shows tour thinking of them and yet keeping it cheap but personal…
Another thing you could get if they love their house/area is a jigsaw puzzle that’s centred on their house from the ordinance survey maps company. Fun thing to do over Christmas and for them to keep that’s personal to them. Depending on how much they like to do jigsaw you could get them a jigsaw roll to use to make it on for eat off doing.