Can I enter the US if I have a pending K3 visa application.?
Answer:
You can certainly try, it is not illegal to visit while having a K-3 pending, but there are no guarantees that the Customs & Border Patrol will allow you entry.
You can improve your chances of being allowed entry if you can bring with lots of evidence of ties to your home country and proof that you have no intentions of abandoning the K-3 process and just remaining in the US with your spouse illegally. This is usually very difficult to prove, that's why most people with pending applications are seldom allowed entry.
Your best bet is to have your spouse visit you in your country. Much less hassle than travelling all the way to the US only to be turned back.