I believe the gear was fixed down as it was a testbed only and changing the design to incorporate this gear would have badly disrupted production.
According to what I found, this wasn't Bf 109V-3, but Bf 109V-31, a serial F-2 equipped with wide track undercarriage and with a retractable belly radiator, as a test-bed for Me-309http://www.letletlet-warplanes.com/wp-content/uploads/2014/11/Bf109G.pdf(page 5 in a file, page 8 in a booklet)
According to what I found, this wasn't Bf 109V-3, but Bf 109V-31...
Mixing Tamiya 1/100 Me-262 and Heller 1/72 (undersized IMO) Bf-109