I took 200mg of atenolol along with 3 mg of klonopin and 20 mg of ambien all at once. Would this be cause to go to the emergency room? I know it's not enough to kill you but I was just wondering what the severity of it was.

How long ago did you do that and are you having any side effects?
Are they prescribed for you to take all at the same time or different times and
What dosages are you suppose to take?
Can you call a pharmacy to ask them?
I don't know what those amounts and combinations can cause.

If it is not enough to kill you, it is not really enough to go to the ER, since it is not life threatening.

Call the poison control line.

Did you do this on purpose and if so, why

A beta blocker for your heart, a benzo for anxiety, and a sleeping aid.
You already know that what you took isn't life threatening, and if you aren't having palpitations or dizziness by now, you probably won't need to go to the ER.

But there is something coy about your title. You have a contract with your prescribing doctors to let them know if you plan or are even thinking of overdosing on the drugs they prescribed for you. So if you are wondering about the fatal dose for all 3, talk to them. It varies quite a bit with the health of the individual, regardless of the overall stats.