The Sheffield rider was among the quickest riders all session and a late run on his Rob Mac Yamah saw him trading times with Shane Byrne, the pair eventually split by a tiny margin
Harris eventually beat Byrne by .015 of a second to prove that wet or dry the Yamaha rider is going to be a consistent front-runner in 2008.
Byrne again demonstrated the consistency and pace he has shown both here and at
Brands Hatch a fortnight ago.
Completing the session in third was yesterday's quickest man Cal Crutchlow. The Honda rider, along with the leading pair, was some way clear of the field all session, and these three must be favourites for front row starts on this form.
Half a second behind these three were Leon Haslam and Tom Sykes on HM Plant Honda and Rizla Suzuki respectively. These two riders so far unable to break into the top three times this weekend.
Sixth place and a superb performance is Adam Jenkinson on the SMT Honda. The immaculately turned out machine showed glimpses of this form at the snowed off Brands round, but a top six time in this company shows real progress from the former Virgin R6 Cup front runner.
Other stand out rides on the wet Hampshire asphalt are Billy McConnell on the MSS Kawasaki in eighth and a very impressive run from Chris Burns on the Cup class MV Agusta.
Burns has hit the ground running here after making some improvements to the Italian machine between rounds. The Privateer scrap between the Geordie and NW200 Ducati ace John Laverty promises to be a close one.
The two were separated here by just under two-tenths of a second.
BSB Free Practice Session 3: Thruxton: Wet track:
1.
Karl Harris GBR Rob Mac Yamaha
1m19.963s
2.
Shane Byrne GBR Airwaves Ducati
1m19.978s
3.
Cal Crutchlow GBR HM Plant Honda
1m20.300s
4.
Leon Haslam GBR HM Plant Honda
1m20.830s
5.
Tom Sykes GBR Rizla Suzuki
1m20.997s
6.
Adam Jenkinson GBR SMT Honda
1m21.424s
7.
Michael Rutter GBR NW200 Ducati
1m21.480s
8.
Billy McConnell AUS MSS Discovery Kawasaki
1m21.494s
9.
Chris Burns GBR STP MV Agusta (CUP)
1m21.529s
10.
John Laverty GBR NW 200 Ducati (CUP)
1m21.716s
11.
Leon Camier GBR Airwaves Ducati
1m21.864s
12.
James Ellison GBR Hydrex/BikeAnimal Honda
1m21.911s
13.
Simon Andrews GBR Lloyds Jentin Yamaha
1m21.921s
14.
Michael Laverty GBR Relentless TAS Suzuki
1m22.116s
15.
Atsushi Watanabe JPN Rizla Suzuki
1m23.046s