Before we can actually have a conversation about the Impact adding a third Designated Player, we must understand that it may not be possible.
Yes, the Impact only have 2 of 3 DP slots filled. However, it is important to remember that DPs still carry with them a hit to the overall cap ($436,250 regardless of how much the player is paid). It is very likely that the Impact wouldn't have the $436,250 in order to add another DP without shedding salary in some other way (i.e. Jack McInerney trade, Nigel Reo-Coker contract termination).
Assuming, for the sake of argument, that it would be possible, in one way or another, that the Impact were in the position to add a third DP, which position should the Impact target? Here are some ideas.
Adding a DP Central Attacking Midfielder/Winger
A DP winger on the right side could compliment Ignacio Piatti quite nicely who can either play in the middle or on the left side. It may diverge from the traditional pattern of using your DPs down the spine of the formation, but with Didier Drogba and Laurent Ciman as anchors at the top and bottom of the pitch it may be the best place to add talent. This might be especially true after the departure of Justin Mapp. Likewise, adding a DP central attacking midfielder could finally allow Piatti to have the peace of mind that he will be on the left side, game in and game out.
Adding a DP Holding Midfielder
The Impact tried this in 2013 when they signed Hernan Bernardello to a DP contract. However we must take this statement with a grain of salt, as his reported salary was significantly below that of a "true" DP. Nonetheless, adding a large piece in the defensive midfielders spot(s) would solve some issues. First, it would lessen the Impact's reliance on older players currently occupying that spot. Second, and perhaps more importantly, a quality holding midfielder might allow the coaching staff to switch to a formation in which only one holding midfielder is necessary - thus allowing for a second forward. This being important in the inevitable situations in which Drogba is not available for selection (i.e. turf, injury, rest, etc.) or has decided to retire (either at some point this season or for the 2017 season).
Adding a DP Forward
Many teams choose to stockpile their DPs at the forward position. And there are good reasons for this. Firstly, a forward who produces at a high rate makes an immediate and obvious impact on his team. Impact fans saw this with Marco Di Vaio and now Drogba. The Galaxy are successful with Giovani dos Santos and Robbie Keane, NYCFC got production out of David Villa, San Jose leans on Wondolowski, Clint Dempsey and Obafemi Martins are the cornerstones in Seattle, and TFC have their best team ever with Jozy Altidore and Sebastian Giovinco. And the list goes on.
Secondly, and less evidently, Major League Soccer has proven to be a league with a fair amount of parity. This is true amongst teams but also amongst the players themselves. Improving your depth at any given position other than forward is great, but the returns don't seem to be as large as when a team improves their depth up top. For example adding a DP winger currently available on the transfer market and interested in coming to MLS would be valuable, but how much better would a DP winger be than Andres Romero, Lucas Ontivero, Dominic Oduro, and Johan Venegas? How much better would a holding midfielder be, who is currently available on the transfer market and willing to come to MLS, than Marco Donadel, Patrice Bernier, Calum Mallace, and Eric Alexander? The key question is not if they would be better but by how much because DP spots are scarce and need to be used wisely.
Moreover, it may be wise for IMFC management to begin looking for a replacement for Drogba sooner rather than later. All reasonable predictions do not see Drogba with the Impact in 2017, and some don't see him playing all of the 34 regular season games plus Canadian Championship games plus playoffs games in 2016 either.
How do you think the Montreal Impact should use their 3rd DP Spot?