I was considering a new Series 3 Apple watch with LTE, but I never found a definitive answer... Apple claims it works with iPhone 6 and later, is the iPhone SE technically included in that mix?
Yes it works with the iPhone SE. The SE is later than the iPhone 6. It uses most of the guts of an iPhone 6S, that's why it is so fast. Same restrictions as iPhone 6 and above, need iOS 11 or later.