Long Sewn in hair extensions with short hair?

I have shoulder length, naturally quite thin hair.. i have been wearing clip in hair extensions for a while but i use the ones with a lot of separate pieces, so i can layer it and make it look natural. But i have decided to get them sewn in, and have purchased 24" really thick extensions, and i am getting three rows, but i am really worried that my Natural hair will show at the bottom and look really silly due to only having three rows:/ my friend has quite long hair, an has her extensions sewn in, an they look really Natural, but im really worried because my hair is so short:/