It's hard to pick three.
The US river system is the Mississippi (and tributaries, including the Missouri and the Illinois and the Ohio and the Arkansas and and and). It's certainly got to be one of the three.
Now, though... it gets harder. There's the Colorado, the Columbia, the Rio Grande (slightly more than half of which is actually on the US-Mexico border), the St. Lawrence (most of which is in Canada, though for part of its course it forms the border between Canada and the US), the Hudson, the Savannah, the Susquehannah, the Connecticut, the Yukon... pick two. In terms of volume, and only counting those rivers that actually discharge into the ocean, it's the Columbia and the St. Lawrence... if you want to disqualify the St. Lawrence as being primarily a Canadian river rather than a US one, then replace it with the Yukon (also partly Canadian, but it does flow through a substantial portion of Alaska also).
The longest and fastest flowing river in Scotland is the River Spey!
it is the Columbia river.
The Mississippi River is the primary north south flowing river in the United States.
Columbia
The Amazon river
Landa River
The River Swale is considered to be the fastest river in England, and it tends to flood a lot as well.
Probably not, but it is the fastest flowing river in Scotland.
Edisto River
Edisto River in South Carolina
rio grande
possibly the Mackenzie but that was just a judgment off a map.