Has anyone had any experience with Tom Lowe’s 3 barrel? I watched a you tube clip and it would make sense running off of a one barrel and then two more kicking in when needed.

Yes, I have some experience, although I'm currently running a four barrel on my 250.

The engine is a .040 over 250 with 250 head. Cam is Tom's Thrifty cam with 1.84/1.6 valves and roller rocker arms. I bought a factory rebuilt Holley 0-80457SA 600 cfm carb. The engine started immediately when I went to break it in. I ran well as a matter of fact until the secondaries opened. I tried the two stiffest springs I could find, metering block with bigger jets than the fixed metering plate that cam in the carb, but nothing worked. It would fall flat on its face on the top end. I talked to Tom a couple times but we couldn't figure out the problem.

Eventually my engine guy said to return it to a 4 barrel so I took the plug out and removed the epoxy. As I was putting it all back together I found that the screen on the fuel inlet fitting was full of what looked like paint chips. I have no idea how I could have put them there since there are two see-through filters on the fuel line. The only thing I can think of is they were in there from the Holley rebuild, but I will never know for sure because I didn't check it before installing it.

From the time I did have the 3 barrel running I will say it had good throttle response and ran cleanly up to the mid range. That being said, the 600 runs really well too...gas mileage be damned.

I have run one.
Found that the jetting was ok as is. But had to slow the secondary opening rate down. Accomplished this by installing a quick change secondary spring housing. Then played with the springs to slow the rate. Black (stiffest) and yellow combo of 2 springs seems to work well for me on the 250.

Resurrecting an old thread with an UPDATE. I was given a Holley 3310-2 (750 cfm) so I decided to try the 3 bbl setup again. I bought another conversion kit from Tom and installed it along with a Quick Fuel adjustable vacuum secondary diaphragm, and a Proform secondary conversion plate (to make it like a 4150 with adjustable jets). I did a lot of tweaking on the idle restrictions, air bleeds, main jets, etc. and learned a lot about Holleys. The only thing that is still black magic is how the emulsion jets work but I have a Proform primary metering plate with a 4 circuit emulsion setup to try in the future.

This configuration is running really well. The low end is much cleaner that with the 4 bbl 600 cfm carb. Secondary actuation is easy to tune-I ended up with a black (stiffest spring) and the adjustable diaphragm turned in most of the way in to slow it down enough. I'm very happy with the results and glad I gave it another try.