I am planning to order replacement ceiling fixtures for my 2 foyer lights. i want to make sure they will work first. measurement wise, the dimensions seem to fit the space well. the ones in there now are standard, came with the new house. in the current "mushroom" light fixture, there are two 60 Watt light bulbs. My new fixture says it will take 100W light bulbs, 3 of them. Does this matter that after I change out the fixture, i will utilize essentially 300W where there now are 120Watt total?
Yes it could matter your best course of action would be to determine all loads connected to that circuit and potential loads to see if the circuit will be likely to handle the additional load. Connected loads will most likely be other lights. You can either identify them and add the wattage or better turn them all on and check the amperage that breaker with a clamp on ammeter then add any general use outlets on that circuit at 180 watts each. A 15 amp circuit should not have more than 12 amps (1440 watts) continuous load a 20 amp circuit no more than 16 amps (1920 watts)
In most cases, this would pose no problem.
Lighting circuits in most homes are 15 amp lines. That means the line can theoretically handle about 1800 watts at one time without tripping the circuit breaker or posing a hazard. That equals 18 100 watt lights at the same time. Or one 1200 watt vacuum cleaner and 6 100 watt bulbs. At the same time. Wattage should be limited to slightly lower levels if the use is continuous.
If you have very few breakers (more fixtures and outlets on fewer circuits) you might have a problem. On most homes built (or rewired) in the last forty years, it probably is not an issue.