band1[k_] := x^2 /. FindRoot [P*Sin[x]/x+Cos[x]-Cos[k], {x, 2,3}]

band2[k_] := x^2 /. FindRoot [P*Sin[x]/x+Cos[x]-Cos[k], {x, 5,6}]

band3[k_] := x^2 /. FindRoot [P*Sin[x]/x+Cos[x]-Cos[k], {x, 8,9}]

P=0.3
Plot[{band1[k], band2[k],band3[k]},{k,0.001,9.42}]

P=1.0
Plot[{band1[k], band2[k],band3[k]},{k,0.001,9.42}]

P=3.0
Plot[{band1[k], band2[k],band3[k]},{k,0.001,9.42}]