Barnstaple beat Camborne 44-13 at Pottington Road to take over at the top of the South West Premier Division table.
Weston, Maidenhead and Exmouth were ahead of Barum going into round four, but the former dropped a point against Drybrook and the latter two lost to Ivybridge and Newbury respectively.
Camborne were only 20-13 down at half time, but a half-time pep talk from Barum captain Winston James turned it into a one-way second half.
James said the eventual margin of victory was the biggest plus point to come out of the game.
"Make no mistake Camborne are a good side so to win as well as we eventually did was a big thing for us," said James.
"Our energy levels were low in the first half and when we went in at half time I was fuming as we had not done much of what we set out to do.
"We had a chat and in the second half we ironed out the problems.
"Once we got our set-piece going, we got going and in the last 20 minutes they could not really live with us."
Barum were never behind in the game after Jake Murphy kicked them in front with a penalty.
By half time Murphy had converted his own try and added a second penalty. Camborne also gave away a penalty try.
Jake Champion kicked a penalty for Camborne and Rhys Brownfield converted a Declan Matthews try and a penalty right on the half-time whistle.
Murphy converted his own try for Barum after the break. Brendan Darley and Luke Berry added tries that went unconverted.
James had the final word for Barnstaple with a try in the last minute. Murphy added the goal points.
Barnstaple travel to Drybrook this Saturday, a fixture the Chiefs will be wary of after a 27-19 defeat at Mannings Ground back in February.