Jigsaw racing do triple Weber manifolds for 6 cylinder Triumphs -
http://www.jigsawracingservices.co.uk/aspiration.htm
You'd need to check for clearance though, three Webers are going to take up an awful lot more space than two Stombergs.
An interesting alternative would be the GoodPart manifolds for triple Stomberg 150's or 175's, available from across the pond -
https://www.goodparts.com/shop/index.php?categoryID=2
It does look like you'd need a mk2 engine and/or cylinder head to fit those though.
I'd be budgeting something like £600-£700 for the Stromberg set up (new manifold, 2nd hand carbs + carb service kits) or between £800 and £1,500 for Webers depending whether you use new or pre-owned carbs.
Alternatively, the Triumph lump responds pretty well to conventional tuning. Phil J's 2.5 six with mk2 vitesse cam and extractor exhaust has 128bhp at the flywheel on Stomberg 150's. God only knows what he'll get from the triple Webers he's fitting to his race car...
Oh, and I have a pair of 175 Strombergs with Vitesse manifold adaptors waiting in the wings to fit to mine in the Autumn. I'll keep you posted on the results.
