“i think 3 share masts with o2 but could be wrong………… “
No. Three own 50% of a network sharing joint venture with T-Mobile – it’s called MBNL. Both networks have c13,000 jointly owned / operated 3G sites giving them more 3G coverage in more places than any other network – Three have c97% population coverage compared to under 90% on both Vodafone and o2.
Since Orange and T-Mobile were merged to form Everything Everywhere, Orange has become a partner in MBNL which in time means even more 3g cell sites.
Separate to that (it’s all a bit complicated) Three have a network roaming agreement with Orange for 2G, which means that if you’re outside of Three’s 3G coverage, you roam into Orange for free.
In practice, as others have said already, Three’s 3G coverage is actually pretty good – in places I go (London, Somerset and the North East) I’ve found that I consistently have 3G coverage on Three when my work vodafone phone only has 2G.