Williamson prepared Pyramid Monarch (driven by son Brad) to win the 1-win mobile trot, and sat behind Galleons Triumph to run third.
Stablemate Monty Python (driven by son Matthew) split the pair to give Williamson his second training trifecta, after Leighton Hest led home a Williamson clean sweep at Oamaru in January 2009.
His oldest son, Nathan, was three lengths off finishing fourth driving Given, which would have given the Williamsons a driving First4.
''We did get a big thrill out of the trifecta - that was nice,'' Williamson said.
Pyramid Monarch recorded her second win in four starts, putting her 10th on the Harness Jewels leaderboard in the 3yr-old trotting category, but the first Saturday in June is not a locked-in target for the Monarchy filly.
''We'll just carry on with her,'' he said.
''If she gets into the field, probably at the time we'll look at it, but it's . . . not a really big goal at this stage.
''There are others there that are just in a different league. I suppose, though, if she's won the money, the owners would probably like to take her.''
Williamson found out how good the 3yr-old trotters are this year when he lined Monty Python up at Addington earlier this month where he finished a battling sixth behind Habibti and Royal Aspirations, which is why he will giving the features a swerve with Monty Python as well.
''Monty will be keeping away from the good 3yr-olds. They're just too good at this stage.
''We'll just race him in the one-win grade - he's on a freebie [a penalty-free win] if he can win one.''
Miss Pegasus won the 2-6 win trot at Ascot Park, and Williamson will be pleased if he can gain a few more before the daughter of Pegasus Spur and multiple group 1 winner Merinai goes to stud.
''She'll go to Addington this week, so that'll be a nice stake for her,'' he said.
''She's a lovely mare and we're thrilled with her. She's done a good job.''
Miss Pegasus has had five wins from 23 starts.
