Can I use a charger with less Ampere than the original?
Maybe. But from what you're saying, the new charger has MORE Ampere than original.
So your 3A supply should work. You need to simply match the voltage at the output stage and provided the current rating is equal to or greater than what you have, it should work fine.
Others haven't mentioned this but you also need to make sure the connector polarity matches. Some are positive centre tip, some are negative centre tip. The after market power supplies will usually come with a range of adapter tips so make sure you choose the correct one. If the laptop is well designed even getting it around the wrong way will not cause any damage.
If the current rating is less than the original 2.5A then what you could do is put extra strain on the power supply to the point where it may overheat or fail to work at all.
In every case the laptop will only draw what it needs (or try to). Current draw on the power supply will actually vary depending on what the laptop is doing. e.g. charging vs, not charging and whether you're doing work that requires a lot of power like playing graphics intensive games etc.
Normally, the amperage rating for the power supply is chosen to be slightly higher than the maximum draw from the laptop. Which means you could even get away with a slightly lower amperage supply, but I wouldn't choose one less... it's asking for trouble.