T3 is actually an international terminal and some of the domestic flights like vistara, air india and jet airways operates from here. So all you have to do is that you have go through the transfers and you will be able to collect your boarding pass for the international flight.

While Heathrow Terminal 3 (T3) is a major hub for international travel, it is not exclusively for international flights, although domestic options are extremely limited compared to Terminal 5. T3 primarily hosts long-haul carriers like American Airlines, Delta, Virgin Atlantic, and members of the oneworld alliance such as Qantas and Emirates. However, because Heathrow serves as a massive connecting hub, you may occasionally find "domestic-leg" flights or short-haul European hops operating out of T3, especially those codeshared with international partners. For example, some British Airways flights that connect to long-haul partner routes may depart from T3 to help facilitate easier passenger transfers. That said, the vast majority of T3 traffic is indeed "International," serving major cities in the US, Asia, and the Middle East. If you are flying within the UK (to Edinburgh, Belfast, or Manchester), you are almost 95% likely to be in Terminal 5 or Terminal 2, but you should always double-check your boarding pass, as airline "terminal shuffling" is common at Heathrow to manage gate capacity and maintenance schedules across the airport's four active terminals.