Sorry no one got back to you sooner. If you are using vetsulin and your cat won't eat, you did the right thing not giving the insulin, especially when you are not home testing and you have only just started the insulin last week and you don’t know how he is reacting to the insulin. Three units is a large dose for a newly diagnosed cat. Vetsulin has a harsher onset than the gentler insulins such as Lantus and levemir, so you need to have some food aboard to counteract the insulin. Do you know why he wouldn’t eat? Does he normally have a good appetite? It is not good to miss doses of insulin so try and get him eating so you can at least give him some of the dose.
However, when you go out to buy the glucose meter, I would also buy a bottle of KETOSTIX which can be found at pharmacies and possibly Walmart for about $10. It is very important to check for ketones in the urine especially when your kitty has an infection such as the UTI Finn is being treated for, is not eating and is having to skip the insulin. The recipe for ketoacidoosis (DKA) which starts off as ketones in the urine is not enough food, not enough insulin and an infection or inflammation so please go out and buy the Ketostix as soon as possible. To test, collect a urine specimen from Finn, dip the test strip in the urine and follow directions as to when to read the result. I don't want to alarm you but this is something that can be avoided if you test for the ketones.
Please let us know how you go with it all.