I take metformin, lisinopril and simvastatin. I am having constipation since last 1.5 months, whose side effect could it be?

Most likely the . Simvastatin as almost all the cholesterol lowering drugs lead to constipation. Usually drinking more caffeine free alcohol free fluids will correct the problem and allow you to take the meds you need for your hypercholesterolemia.
None. None of these meds cause constipation. In fact meteor in tends to cause loose stool.