I'm in the same boat. I actually just returned my refurb AW2 today (it has a chip out of the bezel which pushed me to just returning and waiting).
I unfortunately have to carry two phones (1 work 1 personal) so I have an android & an iPhone. I have a Moto 360 first gen watch now i've used since release. I plan on upgrading my personal phone to an iPhone 8 and pairing it with an AW3 when they hopefully both release in the fall.
I don't think we will have an LTE model later this year, Apple is going to focus more on battery life and the backbones of the independence of the watch before that last step of cellular data because is such a battery drainer.
If other vendors start putting LTE into their smart watches, and in the end if their products don't suffer from excessive battery drain, then it is a good indication the next AW would incorporate LTE. But, the other question is, the billing aspect has to be resolved first, I mean, customers would be reluctant to add another monthly bill on top of their mobile phone bill if you get what I'm saying. This arguably is a tougher problem to solve than any technical ones regarding battery drain and size of such LTE chips being squeezed into an already tight space.
If you really want an Apple Watch (especially for fitness), and you don't have one yet, I'd say buy one today. It's such a game changer for the things it does well, like fitness, that I think you can get a great value out of it from now until September.
Of course, we don't even know that a Series 3 will come out in September. It could just as easily become available in Spring of 2018, and then you will have waited an entire year!