What I would do if I really liked that particular bike:
I'd order a new wheel and shifter with a Nexus 7 or 8 speed hub, install that, then sell the 3 speed wheel and shifter on eBay or Craig's list.
If I was only moderately excited about that bike:
Sell the bike on Craig's list and buy a new one with an 8-speed hub.
I have a bike with the Nexus 8 and have ridden a hilly metric in Illinois on in -- it has plenty of range.