Unfortunately I do not have any solid information, but I do have a couple thoughts that might not be all that helpful.
Louisville Fire and Rocky Mountain Fire do a lot of stuff together, including sharing their radio channels. One will very frequently hear Louisville units on the Rocky Mountain Fire TGs on DTRS. I would not be surprised if this extended into their VHF stuff. It is entirely possible that they relabeled the Louisville's 155.385 MHz to "Tac 3" on the radios to fit in with RMF 1 and 2.
My other theory is that they are possibly talking about one of Louisville PD's not yet used or identified TG on DTRS, which I think are 9562 through 9564. I highly doubt this since they are meant to be used by law.
Hopefully someone else who does know the facts will be able to chime in.
WQUT583 has three frequencies licensed to the city – at city hall
Two repeaters, 159.315 and 161.850
One mobile, 157.250
The 161.850/157.250 pair is a common fire interop pairing in Colorado
Not sure what input they would be using for 159.315, but pretty sure I heard the FD talking simplex on this channel so that may be their TAC3 not on repeater. ULS License - Public Safety Pool, Conventional License - WQUT583 - LOUISVILLE, CITY OF
Don't think they are on 800, all portables still appear to be VHF and agree that they are not likely using PD TG's.
Would be interested in hearing what others have heard on these frequencies.
FWIW (i.e. - shot in the dark), quite a while ago, Mountain View Fire had a Channel 3 on their radios for 153.785 (110.9 PL only on TX).
Spitfire reminded me that it was a Weld Co. Fire Dispatch freq primarily before DTRS.