a boy in a blue uniform is standing next to a boy in red and a boy in yellow and they are holding baseball gloves.
Three boys are holding gloves.
The boys are holding baseball gloves.
A concise way to rephrase the colorful boys and their baseball gloves is simply to say that the three of them are holding gloves.
A boy in a blue uniform, a boy in red, and a boy in yellow qualify as three boys and they are holding a baseball glove which is a form of glove.