There are quite a few, but most are in short supply or completely out of stock as AM3 or even AM3+ platform is old and outdated now.
Try looking for AM3/AM3+ "combo" motherboards instead of "pure" AM3 ones - but even THERE options are getting very slim as most of the folks that used to make those motherboards no longer do so.
https://www.newegg.com/Product/Product.aspx?Item=N82E16813145022 will get you to 5 slots and is low-cost in-stock right now - I've got one of those running a mix of 1070ti 1070 and 1080 cards in a 5-card riser rig right now.