What would you do with a Promissory note with Ex-tenant?

[Sam Y.]

Hi BP community,

 I would like to get your opinion on a promissory note from my ex-tenant, It's like an IOU.

So the story is in California  back before 2009, I had an ex-tenant (single mom+kids) that I was too nice to and I didn't evicted them while they were making late payment month after month.  The back-rents accumulated up to $3400 after several years they living there. In 2009, they finally decided to moved out of the house due to finding another employment elsewhere out of the state. The back-rents still couldn't be paid I so made them signed a promissory note like an IOU to give them 5 years to pay it off at a very low monthly payment.  They made 2 low payments and the check stopped coming in. I never followed thru at the time lacking of knowledge, motivation, busy schedule etc.  Now with more knowledge learned from BP here, I want to go and try to get those back-rent back.   

Does anyone know if this is possible that I can still go after this ex-tenant for the back-rent or too late?

1.  Promissory note says that all balance should be  paid off by end 2013, started in 2009, but now it's 2016 and 3 years later.

2. Should I give this note to a collection agency to help collect? or should I find this ex-tenant myself and try to get the back-rent back myself? I still have their personal info.

3. Bring them to small claim with promissory note they signed? maybe it's too late due to statue of limitation?

The only thing I hate about this is that the tenant didn't kept their part of the deal. This is just help creating bad tenant.  

Should I just cut my loss and move on? What would you do?

Thanks in advance