Give Raffi a call at FSWerks. I got my 15mm for under $120 for a set of 4. May have gone up in price since 2016. I had the longer studs put in as well and no problems ever!
Ya Raffii does not have 3mm . 3mm is a pretty custom size most people do not make or stock 3mm . Most start at 5mm . In fact you can get 5mm spacers from just about anyone all day long for cars.
Most of us want 3mm because unless you have close to stock offset wheels a 10 or 15mm spacer is going to put us down in the 25-32 mm offset range which is really not good for turn in and or wheel well clearance.
He has Dekagrams like I do I believe and they come with a 40 offset already. A 5mm offset would drop the offset to 35 which is lowest one can go without possibly affecting turn in. Even at that offset most likely turn in will be affected.
Most people really love the turn in on our cars stock and when they start getting away from it are not happy.
This is why I went with the lowest possible size spacer for my Caliper clearance I could find. I dropped my offset to 37 from the stock 47. I really would hesitate to go lower. The only other way to go lower would be if one was doing flared fenders and going with 9 inch wheels which is trading off turn in for higher ultimate grip.
It’s all a balancing act and what one is going for and or likes . I will take turn in on a street/ canyon car vs an all out track car where higher ultimate grip might be more advantageous again it’s really up to the driver.