The Tamron is a bit better for close-ups than the 75-300mm, which gets a bit soft. Also, it is much better around 70mm than the 18-70mm is, but the 75-300mm is pretty good at 75mm. I'd rate the Tamron better all round from 18 to 150mm, but the kit tele zoom is better from 150-300mm. It is faster, too.
I would not swap unless I needed a single lens for a special reason. The kit lenses are very light and they perform surprisingly well.