The actual density of the ND filters you have is stated directly. With some ND filters, the strength is expressed in filter factors (2x, 4x, 8x, etc.), and others are expressed in plain-ol' stops (ND-1, ND-2, ND-3, etc.). With the filters that are named for the actual densities, all you need to know is that each change of 0.1 corresponds to the change you get when changing exposure by 1/3 of a stop. Therefore, 0.3 filters are one-stop filters, 0.6 filters are two-stop filters, the 0.9 is a three-stop filter, and so on and so forth.
So, you know that 0.6 can be thought of as a two-stop filter, and 0.9 can be thought of as a three-stop filter. When stacking filters that are expressed in stops or in density, as opposed to in filter factors, you simply add the number of stops or the density of each filter to get the total: seven stops or 2.1.
To work through why this is, say that your 0.9 is the one that is closest to the lens, and the two 0.6's are on the outside. The light passing through the first 0.6 filter is reduced in intensity by 75 percent; two stops equal two halvings, or 1/4 remaining. The remaining light then passes through the second 0.6 filter, and it is reduced by 75 percent; another two stops equal another two halvings. You are now at four halvings (or 1/2 times itself four times = 1/16) of your original amount of light. This remaining light passes through the 0.9 filter, and loses another three halvings, which is 87.5 percent, or 7/8, leaving 1/8 of the 1/16 left over from the original amount of light to makes it through to the lens. 1/8 x 1/16 = 1/128. How many times do you have to split the number 1 in half to get to 1/128? 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128: seven times, thus seven stops. Switch the order of the filters, think it through like this, and you will see that you get the same result in the end.
So, you have cut seven stops before the light even reaches your lens. Then you have your diaphragm, which is capable of closing down to block even more light.
Also, there is less than 100 percent efficiency of transmission at every glass surface along the light's path, including the glass in your lens. (If your diaphragm's setting was expressed in t stops instead of f stops, this would be accounted for.) When stacking so many filters, you may lose enough liht due to inefficiency to warrant a mild exposure compensation. With that many filters, I would probably add 1/3 stop myself, though if you are shooting negative film, it is OK to err on the side of overexposure a bit.
If you are using 100-speed film in sunny 16 conditions, using f/22 you should have a '60 shutter, shooting unfiltered. With the stack of filters, half that seven times, and you have your shutter speed: '30, '15, '8, '4, '2, 1, 2. Two seconds. Open a fraction of a stop to compensate for the inefficiency of the glass, and you get two seconds at f/16-1/2 or 16-2/3 (depending on whether your apertures are set in half stops or third stops).
To get predictable results, you need to know your film's reciprocity characteristics when you start using exposures this long. There are many threads on this already.
At least is saved my 10-22mm lens. How many stops does that give you?
