Individual kittens can weigh more or less than what I have printed below, so this is only a guide, not etched in any big rocks ...
At birth, kittens should weigh approximately 3 to 3 and a half ounces.
At 1 week weight should be approximately 4 ounces.
At 2 weeks, weight should be approximately 7 ounces - Your others sound in line with this however, at almost 3 weeks old now, they should weigh a little more.
At 3 weeks, weight should be approximately 10 ounces - this is where your kittens should be at this stage.
At 4 weeks weight should be approximately 13 ounces.
At 5 weeks, weight should be approximately 1 pound.
By the end of the 8th week, kittens should weigh approximately 2 pounds.
Kittens who do not gain consistantly should be seen and assessed by a vet immediately. This could indicate a serious health problem. Since you are already supplementing - keep doing that but do it more frequently throughout the day. What are you supplementing with?