I'm using Photoshop 6.0...On the tool pallet where you were talking about click on Custom Shape Tool. Then up on the toolbar hit the dropdown box where the shape is and it will open all your custom shapes. If they are not there click on the little round button with the arrow just to the right and select "Load Shapes"

set some guide lines to reflect the constraining outer dimensions of your triangle, then with the line tool draw your three sides.

The corners of your triangle will be the points where the guides intersect.

Three vertical guides: 1 (left side bottom corner), 2 (center top of triangle), 3 (right side bottom of triangle).

you have to visualize it so you set the guides so all sides are equal or so the dimensions are set to what you want.

Once you have the lines laid down, you can clear the guides and manipulate the triangle as you like...

long way about it, but you could also read up on creating custom shapes to be stored for later use in the Custom Shapes Palette..

luck with it