I briefly saw an XK120 FHC on the California Melee this year. It was pretty hopped up, and featured 3 carbs. I’m just wondering what’s involved with this conversion for a 120? I understand some surgery may be needed on the inner wheel well? Is the improvement worth the hassle of the job? Wondering what carbs & manifold are used. I’m just “Wondering” here. Maybe something for the next 120?
I do not own an XK120, but others that do will soon pipe up
I can comment that having triple SU carbs on a 3.4L would not give any benefit, unless you intended to race it, it may not idle well or gain any power at low to medium RPM, in my opinion
If you wanted improvement, then twin 2" SU may (debatably) give you that, without any permanent mods
In my opinion, permanently modifying your XK120 would probably not increase its value
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there have been a few 1950’s triple SU conversion one car finished up with 3 x H8’s and I have an XK120 manifold where the 2 existing carb holes welded up and 3 new holes. then I have seen 4 then the is the motor in Perth from a speedboat with 6!
The triple manifold was made in england specifically for the racing crowd in the late 50’s. Bill Genader’s 120 was so equipped. I also know where another of the manifolds resides. They offer plenty of fuel at speed but won’t help in regular driving.
Philip Porter, in “Original Jaguar XK” shows several photos of a three-carb XK120 manifold produced in the 1950s. Porter says that a limited number were produced for “special customers.”
Around 30-40 years ago, we had a talented fabricator and Jaguar collector who lived in Iowa. He knew about these three-carb manifolds and built one. About 15 years ago, I came across the manifold now in the hands of another local Jaguar parts collector.
Careful: them’s fightin’ words to some people in here… 
These are figures from Paul Skiletter’s Jaguar Sports Cars. They don’t involve 3 x carbs, but do show what kind of improvement to expect from various mods. As mentioned by others, 3 big carbs probably won’t give much improvement if you are still running an A Type head as the breathing is not sufficiently good to benefit from it. IMO, 2 x 2" SUs and a B Type would give the best result relative to cost and complication. Others will probably know, but I believe fitting 3 carbs is complicated by the location of the steering column, perhaps alleviated by fitting an XK140\150 rack and pinion, but that introduces more cost and complication. B
ut, at the end of the day, anything is possible, depending on budget and inclination.
