Good work! The improvement between attempts is impressive! Regarding the second snail, there are some points I think are worth addressing - Add the top-plane of the piece of wood. Currently it's a square and not a box, which makes it look like the snail is floating above it; - Try to avoid the cross-contour on the shell. If you do want to keep it - I think the lines need to be significantly lighter than the contour lines; - I think the shape of the snail's head reads clearer if the pointy things are added onto it, and not a continuation of its contour lines.

I like the way the pear looks. It does seem a bit flat (as opposed to rouneded), but it clearly is standing on a plane and has shadow. As for the shades - the dark shadow and light shadow seem clearly separated to me. But then the dark half-tone is almost the same value as the light-shadow. I'd work on separating those. Maybe draw the values you're aiming for side-by-side on the page, as squares, to see the gradation and work on that a bit? I'd also try doing without the outlines, as they create false separation between values. One last thing - I don't know if it's the picture or the page, but the page looks kinda dark. If it's not white paper, I highly recommend drawing on white. I tried drawing on toned paper, and it's so much harder to get the values right!

The improvement from the first to the other two is really impressive! One thing I'd still try to work on is avoiding the outlines. They are still visible on the lighter parts of the pear in the 3rd drawing.