i modified my code a little bit like the following
a = 1;
k1 = 2;
k2 = 1;
s = NDSolve[{y1''[x] + k1*y1[x] == 0, y2''[x] + k2*y2[x] == 0,
y3''[x] + k1*y3[x] == 0, y1[0] == y2[0], y2[a] == y3[a],
y1'[0] == y2'[0], y2'[a] == y3'[a], y1[-3] == 1,
y1'[-3] == 0}, {y1, y2, y3}, {x...