The rate of melting depends on how alike the particles (small components of the ingredients in these desserts) are with respect to each other, and how strongly these particles interact with one another.

An example - ice made from pure water tends to melt slowly at room temperature because water molecules (particles of water) attract and interact strongly with one another. Once the molecules form connections with one another at low temperatures and become ice crystals, it takes a lot of energy to disrupt these connections (which we can also call bonds) and therefore melt the ice. By contrast, fats such as cooking oils do not have particles that interact very strongly with one another, and therefore require less energy to go from a solid to a liquid. Complex mixtures such as ice cream take a little more thinking.

All three of the desserts you mentioned have some dairy product, which is not a pure liquid at room temperature. They are water mixtures, and water mixtures, compared to pure water, tend to freeze at lower temperatures because the bonds between molecules in the mixture are not as strong as the bonds between pure water molecules. If we go a step further with this logic, these mixtures should also melt faster than pure water ice because of these weaker bonds. However, it is not clear from the information above which of the three melts the fastest because the rate of melting would depend on the components in these desserts.

Typically, ice cream contains more percentage of dairy than ice milk, which typically contains a lower percentage of dairy than sherbet. Therefore, you may expect that ice cream melts the fastest and sherbet melts the slowest. However, there may be differences in the amount of dairy in, say, ice cream from different companies/brands, which would affect the melting rates.

I highly encourage you to test these principles by buying all three types of desserts from the same brand, measuring out the same amount for each type, setting these at room temperature, and recording the time it takes for the three to melt.

Is this going to be your science fair project? Here are links by people who did similar experiments:
link 1
link 2
link 3

You should act like a scientist and do the experiment yourself!

My prediction is that it will be ice cream, because it has (I believe) a higher fat content than the other two, and fat has a higher melting point than water.