Your Hoya 25a has an 8x filter factor. They also say "3" which is simpler to think - 3 f/stops.

So with a Red filter you open up 3 stops normally (from a meter reading without filter).

I may have opened the can of worms in your mind when I described the different effects you can get with a filter and the different exposures that would lead to those results. I said if you want to make green normal while really brightening red you would open more. Strictly speaking you can perform any of the exposure variations in the darkroom when printing. You don't have to wrangle with the crazy thought I threw out there.

For your example, the in-camera meter told you 2 stops. Well it was off the filter factor because it was about to make you underexpose 1 stop. You probably should open up at least one more stop.

There is a refined "Hutchings" method in "Steve Simmons Using the View Camera" - Amazon will serve the specific page if you are lucky but avoid untrustworthy spyware download sites in Google results. In the Hutchings method you meter through a filter and then compensate according to a short chart.

In that chart a 25a red filter says open up yet additional two stops after reading through the filter.

Reason: shadows have a lot of blue light - and the blue light is severely cut by red filter. So to get sufficient shadow detail you need much more light than the meter indicates.

So a factor quoted as 3 meant 3 f/stops. Your meter was a little off. Steve Simmons would say a lot off.

Reality: You are fine following the meter reading but the Hutchings method is easy to use (for many filters it says use reading through the meter).

Strictly speaking: I think it was for Tri-X, different factors might need to be "worked up" to be accurate with tabular grain film.

Back to reality: I carried the Hutchings chart in the wilderness and when I needed it, I was too lazy to turn to the page I had written it on. If you like the concept, tape it to the back of your camera.