import numpy as np
import matplotlib.pyplot as plt

tot_time = 200

time1 = 30
time2 = 90
time3 = 150

depth0 = 0
depth1 = 3
depth2 = 5

depth_des = np.arange(tot_time)

depth_des = np.piecewise(depth_des, [depth_des < time1, (depth_des >= time1) & (depth_des < time2), (depth_des >= time2) & (depth_des < time3), depth_des >= time3], [depth0, depth1, depth2, depth0])

plt.plot(depth_des)