Photos are underexposed?

Ok, so I was using my square and filters which i stacked 3 together(0.3,0.6,0.9) and i was taking photos on a sunny dayof flowing water. When i used my cameras settings on aperture priority mode my photos came out really underexposed (like nearly black) and when i went on auto settings the same thing happened! Why is this happening? How can I fix this? How can I stop it from happening?!
PS sorry this is my first time using and filters

Added (1). i'm using a nikon d3300, viewfinder and no exposure compensation

I have more questions than answers. Why did you post this question anonymously? Do you have exposure compensation dialled in? Which camera are you using? Are you trying to use a Nikon DSLR in live view?

Update: Then I have no idea, from the information you supplied there's no reason why it shouldn't work. The only other thing I can think of is if you are using a manual lens (non CPU), your camera can't meter with one of those. If so, use it on manual mode, and experiment until you get the correct exposure.

You should use bulb setting the 30 secs in av mode was too short