Hi jaishreema.
While you can buy a firmer topper and add it to the current matters, it won’t make the mattress more supportive or fix any deep support or alignment issues because the topper will just bend into the softer layers below. Typically, adding a firmer topper to a softer mattress would work only for those that are looking to give their current mattress a crispier feel so that they don’t sink into it as much and don’t need to fix deep support. You can find a bit more about pressure relief and support, about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
It sounds like Eco Terra’s medium firm option may have been a better choice for you and I suggest that you contact the company to see if they would be willing to exchange it for the other comfort option they offer, If the medium-firm option will be too firm it would be much easier to fine-tune and fix it with a topper without compromising alignment. You can read more about their refund policy here
Depending on your needs if you wish to try adding a topper then there is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
Another option that I would not recommend would mattress surgery which involves removing and replacing your 3" Talalay comfort layer, or generally other components that are too soft (instead of adding to them) which in most cases can be much more difficult to do.
Phoenix