Difference is everything but the bullet diameter. The chamber shape, the brass construction, the shoulders, everything is totally different. You cannot, shouldnot try to insert any of these in anything they are not chambered for. 300Win Mag is pretty powerful and shines with 150-200gr bullets and generally is 100-200fps slower than a Weatherby Mag. A Weatherby is a larger case, but has a unique shoulder rounded shape and is belted like a win mag. The 300Wthby is generally 100-200fps faster than the Win mag. Another thing unique about the Weatherby is freebore. You cannot seat the bullet to touch the lands and grooves in a Weatherby. The Remington mag, I assume your talking about the Ulltra mag, has more case capacity than the Weatherby, and about 100fps faster than the Weatherby. Unlike the weatherby, it has no belt on the case, and much less freebore but still some.

I mean absolutely no offense by this. But if you have to ask, then all three is a little too much cartridge for a new shooter.