If you were served Sept. 21, you have 5 days to respond (so Sept. 26 is the last day to file something).
However, if the landlord has not yet filed a request for entry of default, you can still get a responsive pleading in on time.
The California Courts have an excellent website that walks you through the entire process and gives you all of the forms you need to present your defense. You can find that here: http://www.courts.ca.gov/selfhelp-housing.htm
Unfortunately if the basis for the eviction is non-payment of rent, and you did not pay the full amount of rent due within the 3 days, the landlord has no obligation to accept your rent now (even if you offered 100%). The court may "encourage" the landlord to enter into a settlement with you - but again, the landlord is not required to do so.
If you do not have the full amount due, and there is a history of non-payment or late payment - the landlord's incentive to settle is greatly reduced.
If you find that you are evicted - despite filing an answer, delaying the matter through a trial, and offering a settlement - contact your local (county) housing authority, advise them that you have been evicted and they will place you at the top of their housing list (particularly your roommate with the minor child).