Three UK PAYG sim for USA Roaming

I will be travelling to the USA later next month, last time I was over last year I paid $50 for a T-Mobile Simcard with data, calls and texts, it was an absolute ripoff and we are very lucky in Ireland to have affordable connectivity here.

Since I am only going to the US for a week I am thinking of getting a Three UK PAYG simcard from Northern Ireland before I depart as I will up there next week,

http://www.three.co.uk/go-roam#faq

I am with Eir myself and plan to utilise the wifi calling system for calling home to Ireland to avoid roaming charges on that. Once I have data its what matters for whats app, uber, google maps etc.

I remember like 9-10 years ago as a former Three Ireland customer being able to use Three just like home to roam in the UK, EU (before roaming abolition) and Australia etc. but Three here have since withdrawn that but Three UK still offer it to their PAYG customers.

I'm thinking what is the procedure for getting a Three UK simcard since I don't have an address there, I could top it up with £5 to £10 whatever is the minimum I could get away with. If I need to make any calls within the US I could always use Skype-out as I use that whenever I dial international anyway even now.

Has anyone done this before to avoid the ridiculous roaming charges and exorbitant US mobile network pricing?