You do need to change your feeding practices. From your description, it sounds as though your cat is being overfed, which will just make the diabetes worse.
First – I would change the diet to one appropriate for a diabetic patient, and will help with weight loss. It needs to be low in carbohydrates. There are many veterinary diets out there that would be appropriate, talk to your vet exactly which product to use. It's not something that can be half-done. Of course if the cat does not like the diet, then it needs to be changed to another one formulated for diabetic patients. Trust me that feeding the right food, in the right amounts, is much less costly and stressful than pushing the cat into a diabetic crisis. Getting on the right diet may also allow you to decrease the insulin dose over time.
Purina DM is the most popular diet for diabetic cats in my experience, many clients also like Hill's m/d.
Glucose monitoring is a must in diabetic patients. If you can do this at home it is ideal, using a glucometer by a needle prick in an ear vein (ask your vet for further instructions). A one time glucose reading at the vet is of little use to anyone; the cat is likely already has a stress hyperglycemia at that point so you have no idea if the diabetes is well managed or not. Blood glucose curves are needed for this purpose, whether done by you at home or on an outpatient basis at the vet clinic. Your vet may also recommend fructosamine testing, which gives a better idea of how glucose has been managed over a two week period.
Now getting to the question. You need to alter your feeding cycle. For one thing, I suspect your cat is getting far too many calories being fed wet food four times daily and free choice dry food.
Transition gradually to set meals during the day, no free choice food if possible. This may be a challenge for a cat that has always been a grazer, and your cat may not be happy about it, but you need to control what your cat is eating. Leaving food out all day will not prevent him from becoming hypoglycemic; if it happens he wouldn't think "I need to eat sugar" like a human diabetic might think. He just wouldn't feel good, and would not eat. When his glucose is normal he's just going to keep eating all day whenever he feels like it.
Ask your vet to calculate the caloric needs of your cat based on the diet fed, to make sure you are feeding an appropriate amount.
Different vets will give different recommendations as to when to give the insulin. As cats can be finicky when they want to eat, I always recommend they eat a substantial amount of their meal before any insulin is given. My recommendation is to give the insulin while the cat is eating or within 30 minutes of finishing the meal. I would be cautious giving it before a meal as you cannot be sure how much the cat will eat.
On lower doses of insulin it is less of a concern that a cat will become hypoglycemic, but when giving 3 units twice daily more thought does have to be given with timing of feeding and insulin dosing.
The insulin product you are using may make a difference as to ideal meal timing. Long-acting insulin, glargine (Lantus), is marketed as a "peakless" insulin, implying that it doesn't have to be given with food, but it does still peak in cats who can become hypoglycemic if not eating appropriately.
As these diabetic cats usually need to be on a weight loss diet, I recommend sticking to twice daily feeding regimen, with insulin given at those two mealtimes. A small third or fourth meal is ok as long as you spread out the calories throughout the day, and give most of the food at the time of insulin dosing.
Any changes in diet should not be done suddenly; i.e. don't go from free-feeding to twice daily meals overnight. Just gradually decrease the amount free-fed, and increase the amount fed at set meal times.
If the cat won't eat it's meal, do not give any insulin to be on the safe side. If it's eaten recently (say in the last 6 hours) then you are probably safe to give a half dose of insulin. If the appetite isn't back to normal within the next few hours then your cat may be sick and need to see a veterinarian as soon as possible.
These recommendations are largely based on clinical experience as well as the ISFM Consensus Guidelines on the Practical Management of Diabetes Mellitus in Cats (2015).
Management of diabetic cats can be one of the most challenging things for vets and owners – best of luck in getting your cat well regulated.