plot1=Plot3D[{120- 2 q-r,60- (q+r)/2,120-  q-2r},{q,0,120},{r,0,120}] 
plot2=ListPlot[{{0,60},{30,30},{40,40},{60,0}},PlotStyle->PointSize[.02]];
Show[plot1,PlotRange->{{0,130 },{0,130} , {0,130}},AspectRatio->1/1/1,AxesLabel->{"Firm 2's Output","Firm 1's Output","Firm 3's Output"}] 




plot3=Graphics[Text["Firm 1's Reaction Curve",{80,30}]];
plot4=Graphics[Text["Firm 2's Reaction Curve",{30,80}]];
plot5=Graphics[Text["Cournot equilibrium",{42,42}]];
plot6=Graphics[Text["Cartel Output",{32,32}]];

Show[plot2,plot1,plot4,plot3,plot5,plot6,PlotRange->{{0,130},{0,130}},AspectRatio->1/1,AxesLabel->{"Firm 2's Output","Firm 1's Output"}];