We have a 3 month old, she is just under 18 lbs, but I am 6'5, 250 lbs, is she gaining weight too fast?

Growth chart. To answer your question one must look at your baby's growth chart. How tall is she now? What does her growth and weight curve look like? Is she gaining more weight compared to height or are they "evenly increasing"? You need to talk to your pediatrician about all these. There is a broad range of normal. Your baby's weight can only be interpreted (fairly) w knowledge of all other parameters.
Yes! Normal weight gain for a full-term born infant would be 1 1/2 - 2 lbs per month during the first 4 months. Therefore, 18 lbs at 3 months of age is not normal by any means.