I am currently on week 3 post gastric sleeve op and tbh I'm not a massive fan of puree food so been sticking to milk and soup. I've been away for a week so excited to weigh and it was only 1lb yet hardly eating anything. Could not eating enough cause weight loss to slow down? (Lost 26lbs prior)

It couldn't.

You are loosing fat, but maybe have some excess water weight, probably some swelling from surgery that shows on scale.

Week 3 stall is a common thing anyway.

Generally not - your initial loss these first 2=3 weeks is mostly water weight caused by burning up your short term energy stores of glycogen (basically stored carbohydrate in your muscles) which need water to stay in solution. Once that has been depleted, your body needs to restore the glycogen reserves to some functional degree by converting whatever it can that you eat, or from your fat stores, to rebuild it - which means that it also needs to hoard some water to get that back into solution. So you will hold on to some water weight, or possibly gain a bit, while this happens. Once things stabilize again, your scale weight will start to drop only now your body will be drawing from its longer term fat stores rather than glycogen - which is what you want! Note also that as this happens your loss rate will probably drop some, as the fat takes more calories to burn than the glycogen, (on the order of 3500 calories per lb vs. 2000 calories per lb) but you are doing what you came here to do now.