Very likely they will sue
Chances of getting sued for a $3K credit card debt are 100%. Credit card companies are no longer just writing off and selling bad debt. Citibank is one of the cards that have become much more aggressive in collecting defaulted accounts. If it still owns the debt and has merely hired a collection agency, it is very likely that they will sue.
In general, making payments to a collection agency is bad idea. They tend to accept the payments for a year and then re-sell your debt to another collection agency.
You are always better off making a lump sum settlement. Always get any payment plan or settlement agreement in writing before you pay.
Never give a collection agency access to your bank account. Use USPS money order or cashier’s checks instead.
Absolutely DO NOT just send small payments. That will not keep them from filing lawsuit against you, and only resets the Statute of Limitations, so in fact you’re making things worse for yourself.
See how-to-pay-off-debt.html for more tips on dealing with collection agencies.