In general, yes, filter factors are additive.

Can you trust your meter, when you meter through the filter? It depends. The spectral sensitivity of the meter may be different from the film, and then if you've got a lot of filtration, you may reduce the light getting to the meter sufficiently, so that the response of the meter is no longer linear.

With a circular polarizer, you should be able to meter accurately through the filter, unless the light is very low to begin with. One thing to watch out for with the wide lens and a polarizer is uneven polarization. This is particularly visible with landcapes that have a lot of open sky. I usually don't use a polarizer with anything wider than 28mm on a 35mm camera or equivalent, unless it's a scene that lends itself to that where the polarization effect is needed only in one area.

An ND 3x filter is 1.5 stops (not 3 stops). Is this a solid ND or an ND grad? If it's an ND grad, meter for the land, and then adjust the grad filter to bring down the sky, unless you have matrix metering, where you can probably get a good result by adjusting the filter and then just metering normally.

In general a 25A filter has a factor of 8x or 3 stops, but this can vary with some films. Careful with a 25A. It's a filter that beginners tend to overuse (I certainly did!). It usually is a bit overdramatic. Now I'm much more likely to use a medium yellow or orange filter to improve cloud separation. Another neat trick is to use one of those color grads, like a tobacco grad, designed for cheesy fake sunsets with color, but use it for B&W. It will increase contrast in the sky and bring the exposure closer to the land, but won't affect the land.