3 Units is a big dose to start with. Most cats should be started at between .5 and 1U
You can always go up if necessary, but once it's in, you can't take it out
Also, curves done at the vet's office are unreliable due to vet stress which raises the blood glucose up to 200 points. The only way to know what his BG really is is to home test where he's relaxed and in his own space.