SetDirectory["..."];
data = Import["ABC.dat"] // Transpose;
ci = data[[2 ;; 4, All]];
pfun = ParametricNDSolveValue[{aa'[t] == -k1*aa[t],
 bb'[t] == -k2*bb[t] + k1*aa[t], cc'[t] == k2*bb[t],
 aa[ti[[1]]] == 1, bb[ti[[1]]] == 0, cc[ti[[1]]] == 0},
{aa, bb, cc}, {t, 0, 200}, {k1, k2}];
f[k1_?NumericQ, k2_?NumericQ] :=
 Sum[Total[(ci[[i, All]] - Map[pfun[k1, k2][[i]], ti])^2],
{i, 1, 3}] // Quiet
fit = NMinimize[f[k1, k2], {k1, k2}];
params = fit // Last
Table[Show[ListPlot[Transpose[{ti, ci[[i]]}]],
 Plot[(pfun[k1, k2] /. params)[[i]][t], {t, ti[[1]],
 ti // Last}]], {i, 1, 3}]