Hello,
I have gone through your question and i will do my best to solve your doubts.
First of all, conception can occur when a healthy motile sperm fertilizes ovum, so mainly depending on
healthy sperm and ovum with favorable environment. Precum contains small quantities of sperm, so there are every chances that you become pregnant eventhough semen does not enter into vagina.
Coming to your
irregular periods, its very difficult and almost unpredictable to find out when ovulation occurs and ovum releases. For that you need to do regular follow up with
ultrasound to find out the exact time of ovulation.
In your case, ovulation could have been occurred immediately after your periods or ovulation is delayed for many days, so its almost impossible to pin point your chances of conception.
But there are every chances that you could become pregnant with precum and having unprotected sex after 3 days, so its better to wait for 1 month or so and if your period does not come, then you should go for
pregnancy card test or ultrasound to rule out pregnancy.
Hope this will help you to clear your doubts.