Histograms and time of the project

When i have 4 jobs: 1 job->mean=6 days and std=1 day 2 job->mean=2 days and std=2 day 3 job->mean=7 days and std=2 day.Histogram And job 2 only start when job 1 is finished. I have to draw an histogram of the total time of the project and the days to be completed with 99% of confidence. I have this, but it doesn't give me the total time:

N=10
for i in range(N):
    t1=np.random.normal(6,1,N)
    t2=np.random.normal(2,2,N)  
    t3=np.random.normal(7,2,N)
    # task 2 can only start when task 1 is completed.
    # t is the completion time of all jobs
    t=max(t1+t2,t3)
print t

Thank you