It might make more sense for Three to have n78 in random places than other networks because they also compete with fixed providers. When a PoW goes live, their "home broadband" service also becomes available. It's not just for phones.
Their return on investment is bad, but I have to ask if they're not intentionally keeping it low. No one goes on a spending spree and expects to make a lot of money while selling unlimited business plans for £6 a month.
They've been preparing for the merger for a while and it's in their interest to make their numbers look as bad as possible. They (and Vodafone) even went to
the business and trade committee and told everyone how sh*t their network is compared to EE and O2, something that no network wants to admit... all this helps them with the merger.