Every pup is different. They all grow at different rates. Parental genetics plays a rule. Gender plays a role. Is you pup male or female? How much do the sire and dam weigh?
Pictures of your pup from a standing side view and top down would be helpful.
Most important...unless your pup has had a serious health crisis or very heavy parasite load there usually isn't any need to worry about weight while growing if the pup is eating well and displaying normal puppy activity levels.
Those online growth charts are just averages but in my experience are not very accurate. They create a lot of worry. Did for me when our girl was a pup. Ignore them.
15 Lbs doesn't seem like under weight to me without pictures. My girl was 17 lbs at 12 ish weeks and she grow slow and steady to a 78 lb adult that is probably a tad overweight. She is a bit tall for a female.
Got it! I have a female. Vet check turned out great when I took her in at 9 weeks. Current on vaccines and worming. She looks okay/healthy or at least I think so. Alert. Eyes clear. No drainage of any kind. Nose good. Poop excellent consistency. Ears clean etc. Sire 110 and dam around 85. She most definitely acts and appears normal for a puppy. She’s very energetic and spunky. I was looking on line at some of the weight ranges and was trying to compare those with her current weight. Naturally I worried when she wasn’t in said ranges. Will try to get pictures later today. ?
She looks to be a healthy weight for her age and body structure. I would agree that unless she chooses not to at 12 weeks I would be doing 3 feedings a day. Just divide her total daily portion by 3 rather than 2.