First of all, you must remember that it is impossible to reason with a 3 year old. Save your energy and strength. Second, look at what she is doing. By hiding his toys, she's actually showing displeasure at the fact that he has usurped her position in the family. She was the centre of attention for all of 2 years, and now, she has to share that attention with a baby.
This is a case of sibling rivalry. Remember just because she can't put into words how she is feeling, doesn't mean that she isn't experiencing severe feelings of jealousy. Children have the same emotions that adults have, the only difference between adults and children is a larger vocabulary.
But, you need to teach her how to share. The first step is to establish, common toys. What is that? A common toy is one that they both play with, this is kept separate from her toys and his toys. It will be like that with all things. Forcing a child to share his/her toys with a younger sibling, actually builds resentment and hatred in the older one for the younger one. So you have establish a rule of yours, mine and ours.
Also, telling her that she doesn't need to be jealous of younger brother won't work. Establish separate bonding times. Have activities that you do with her and not with the baby at the same time. Work out a plan with your spouse, and remember, that you have two children with needs. It's common to overlook the older one for the younger one.
But the most important thing that you need is patience. These things will come in time, and only with re-enforcement. Punish her for hiding his toys. Set rules that are simple and don't go into long explanations. No is no. Bad is bad and good is good.