Is it okay to insulate some but not all steam pipes coming from boiler?

Any pipes you insulate, including elbows and tees will help contain heat and make it available to where you want it, in your case, the third floor.

The insulation of hot water pipes has the following purposes:

• Reducing the heat loss so that the hot water at the point of use is not significantly colder than at the point of provision.
• Reducing the heat loss that the provided energy is used effectively and e.g. heating costs are reduced.
• When the system is still and parts are located in a freezing area, the probability of freezing of water is reduced.
• Protection against burn of skin, especially if parts are in a passageway.

Therefore, there are only few areas where such pipes not have to be insulated. Only such pipes are not always to insulate, which are located in the same room as the heater. But piping in a conventional basement should be insulated. Piping in a drywall should be insulated too. Because the heat is often transmitted into rooms which do not need the heat.

The completeness of the insulation affect the effect of insulation. Less insulation cause a less effectiveness of the insulation, e.g. non insulated bends, branches or fittings.