How likely is pregnant if intercourse was taken place 3 days before expected period but cycles are very irregular?

20%: People with irregular cycles also have irregular ovulation. Therefore, i will quote you the average rate of getting pregnant any cycle: 20%. I suggest you pursue a reliable form of contraception until you're ready to conceive.

It's possible: Given that your periods are irregular, it is by definition, difficult to state on a regular basis when you ovulate. Therefore pregnancy is possible. In a women who is regular with her cycle it is less likely she would conceive that far removed from when she would have a ovulated--namely 3 days before her period.

Likely: Unprotected intercourse can lead to pregnancy. If your cycles are irregular, it means that you could potentially ovulate at any time. If you are going to have intercourse and you do not want to become pregnant, you must use birth control (preferably redundant forms; condoms plus pill/iud/sponge/etc.) since no form of contraception is 100% effective.