£1.50 for 3 at BnM

Only 10% smaller so b&m is a better deal

B&M is 580g for 250p = .431p per 1g
Morrisons is 640g for 247p = .385p per 1g

but to be fair the Farmfoods deal is better

http://www.hotukdeals.com/deals/john-west-tuna-chunks-brine-4x-160g-2-39-2-for-4-farmfoods-2413130

Might be worth pointing out that John West are massive gits and bottom of the Greenpeace tuna league table for unsustainable tuna fishing:

They promised 100% sustainable tuna by 2016 - so far they've managed 2%. They show little intention to change.
John West's owners, Thai Union, have been linked to human rights abuses in their seafood supply chains
They must stop using unsustainable fishing methods that catch sharks, endangered turtles and other creatures

https://secure.greenpeace.org.uk/page/s/not-just-tuna-fb

I know a lot of people won't care but there are other far more ethical supplies in that list that are actually cheaper than John West, even with the above deals.