Use a duplex (stacked) switch

The easiest thing to do is use a combination switch like these. They're available in both the traditional toggle style and the more modern decorator style and a variety of colors.

You could also remove the old box and install a new 3-gang box but it's a lot more work.

Since you ask about receptacles, rather than switches, the short answer is 'no', you cannot put more than two duplex receptacles in a two-gang workbox.
You can install a single-gang box closeby, but not close enough to cover them all with one faceplate.
For a combination of devices in your 2-gang, you could have:
- one switch/simplex receptacle combo as one device and a duplex receptacle as the other;
or
- (as previously mentioned) a double switch as one device and a duplex receptacle as the other;
or
- two duplex receptacles

Don't know why I read that as switch instead of receptacle. :(

That's ok Normel, I just followed your lead. :-)

I know there Despard style receptacles but I don't know if they are approved any more.

Depending on the age and type of box you may be able to attach another device box to it.

The old box needs to be an expandable type with screws holding the sides on.
The dimensions for the screw placements have moved around over the years, so it could be hard to find a compatible box to attach.

You will need to make a larger opening and do some wall repair to get the new gang in and attached.

I would remove the old box and move the cables into a replacement 3-gang old work box.

If done very carefully the hole in the wall for the new box will be the correct size and no wall repair will be required.

BOXFILL limits still have to be met. Introducing additional receps, switches, or branch circuits adds to the extant box-fill.

Here in CBS-land, only shallow boxes fit on the inside of exterior walls (just one layer drywall + 1" furring strip over CMU block). Thus you often have to use a double (or quad) gang box just to host a single (or double) recep or switch, esp. if add'l conductors are "passing thru", in order to have enough cubic inches to meet boxfill.

"...only shallow boxes fit on the inside of exterior walls (just one layer drywall + 1" furring strip over CMU block)."

In these cases there are at least two options.

Use 4 inch square boxes with plaster rings to meet fill, or simply cut a pocket into the CMU to allow a deeper box.

Neither takes very long to accomplish.

If the box falls over a web in the CMU it means more material must be removed, but it is soft enough to not be a problem (unlike older cement blocks that are a bear to even drill into or drive fasteners).