I have twin boys who are 6 weeks old. They breastfeed each 3 hours as an average. However this is making me tired especially during the night. Till what age will they keep waking each 3 hours? Is there anything I could do to make them sleep longer durations during nights?
There's not just one answer to that question, because it varies by the child and the situation. Could be 2-3 months, could be much longer (easily could be 6mo+).
There's also a second issue with your question: 'waking up to nurse' versus 'waking up hungry'. If they're waking up actually hungry, then you need to feed them (or find a solution to feeding them in some other fashion). If they're not actually hungry, they may just want to nurse because they like the feeling and the attention. Kids will very often do that for quite a long time, and you need to learn to differentiate waking from real hunger and waking just because they want mommy - and deal with them differently.
The most general answer to your question is that for kids that are 'bad' at waking up, often when they start consuming a significant quantity of solid foods (usually around 5-6 months), they begin to sleep better at night. That's not a suggestion to start solids early, as that's not necessarily good for their development, but more of a 'this is when it may happen'.
You may want to consider pumping if you're not already doing so, and having your husband or another caretaker responsible for some middle of the night feedings, perhaps every few days so you're able to get more regular sleep.