To add to the above post...You are a huge part of his life. You may not see it now but, when he's older and he say, gets picked on at school, has his heartbroken, wins his first game. Trust me you will be the one he comes to for his comfort and support. You are is constant and he will know that.
I know you probably feel it's not fair b/c your missing out on being a grandparent. But, you are getting the joy of raising this child. Stand by him now no matter how excited he gets around his other grandmother. As his primary caretaker he should always feel he can come to you with anything and that you'll love him no matter what.
" I know he is just a child and do not understand, when I ask why he like her so much he just say "I don't know". I sometime find myself pulling away from him but then feel guilty and retract."
First of all...you're talking about a 3 year old child!!!! You even ASKING him that question is inappropriate! The VERY LAST thing you want to do is to cause confused feelings in this little boy as to why he has the feeling he does! And "pulling away from him"? YOU are this boy's whole world...and whether or not he likes spending time with his other grandma (which should make you HAPPY b/c the boy has more than one person who is willing to love and spoil him!).....this should NOT affect your relationship with him, or how you act towards him IN ANY WAY.
I understand that you are feeling jealous and a bit offended...but whatever you do...do NOT make it your grandson's problem in ANY way, b/c that isn't fair to him. YOU are the grown-up. Instead of looking at it like "he likes her better"..embrace the fact that he has TWO loving grandmas and that you've done so well for him. You have the every day hum drum disciplinarian life...and he's 3...his other grandma is his play toy for the most part, which you recognize. Those kinds of people always get that kind of reaction from children. But you are giving him a foundation...a life...which is priceless and irreplaceable for his lifetime.
I'd say basically...suck it up...and if you feel yourself somehow involving him...take a HUGE step back, b/c it isn't fair to him. Allow him to have his relationship with her. His excitement in seeing her is in NO way any indication that he doesn't adore you just as much....he just sees you every day. Believe me...you are vital in that boy's life.