To access the hard-to-reach items on level 3, The Gantlet:

There is a very large, rectangular area dominating the east
side of the map.  Go to the far east side of this area, and
you'll find the blue key on the ground, close to a pair of
blue-keyed doors.
Grab the key and open the left-hand door.  Follow the hall
north and west, to a switch you'll see ahead of you, and
Activate it.

Now head back out into the large rectangular area, and go
to the top of the western-most staircase.  From here, climb
up on the northern ledge that runs west-to-east alongside
the stairs.  Follow this ledge east to a narrow bridge on
your left, which leads north to a teleporter booth in the
north wall.

Walk up to the teleporter, but don't step in.  Turn to face
south, with your back close to the teleport pad.
Now look at your Automap.  Carefully adjust the player icon
so that you're standing on the eastern half of this bridge,
and turn so you're facing as directly south as possible.
(You may want to zoom in the Automap to increase accuracy.)

When you switch back to first-person view, you should see
the bridge stretching out before you, and you should be
standing a little to the left of center.  (In other words,
stand just to the left of an imaginary line running south
down the middle of the bridge.)

Now Run forward (hold the Shift key down in PC Doom II),
and keep running until you hit the southern-most wall of
this large rectangular area.

What should happen is that you run down the bridge, across
the staircase, over the opposite side of the staircase, and
you'll land on the western edge of a wall that runs around
a sunken "well" against the south wall of this area.

If you didn't make the jump, it might be because your
alignment was slightly off, or perhaps a monster got in
your way (players and live monsters cannot cross over or
under one another)... simply try again.

There are also other routes that you can take to jump into
the well; some by just running, others through a combined
use of Run and Strafe.  If one route doesn't work for you,
experiment with others.

Once you land on the edge of this well, simply step inside.
You'll find some goodies there, and a teleporter pad that
takes you back outside.

PC Doom II players can click here to access a demo showing
how to get into the well.