sq[t_]:=Mod[Floor[t],2];
Plot[sq[t]-1/2,{t,0,10}]

saw[t_]:=(t-Floor[t])-1/2;
Plot[saw[t],{t,0,10}]

tri[t_]:=-Sign[sq[t]-1/2]*((t-Floor[t])-1/2);
Plot[tri[t],{t,0,10}]