I am 3month pregnant and during prenatal visit doctor said that my placenta is a bit thick and not to worry.can you please tell any issues with this?

Probably not: At 3 months the placenta can be different shapes. I'm not even sure why this was told to you if your doctor said not to worry!

Usually none @3 mos.: The first trimester is not a typical time for placentomegaly to be diagnosed, although i guess it is possible to suspect it before 14 weeks, but confirmation will likely be thereafter. If confirmed, it is a marker for triploidy (lethal) fetal infections or molar pregnancies. Clarify this comment with your obstetrician and get a maternal-fetal medicine sonogram to elucidate this further.