How to synchronize JMeter Thread groups?

Question

I read a lot of literature about JMeter but don't know How to synchronize JMeter Thread groups? I have 3 threads group:
A thread group with 3 request (1,2,3)
B thread group with 3 request (4,5,6)
C thread group with SMTP Sampler

When I run project - request completed random. For example:
request 2
request 4
request 1
request 3
request 5
request 6

But I want synchronize threads this 2 threads and after run 3rd threads with mail.
request 1
request 2
request 3
request 4
request 5
request 6
request send mail (SMTP Sampler)

how can I do it? can write simple example, please.

Answer 1

To run the thread group one by one, Please select this option in your Test plan