Hi! Actually it should be 이렇게 하는 "것", but 것 becomes just 거 in verbal conversations. 1. 이렇게 하는 거: It should be actually followed by 가 which is a subject particle, but 가 was omitted here. And it's totally fine to be omitted. So, it's 이렇게 하는거(가) 2. 이렇게 하는 게: 게 is shorten from 것이, 것 + 이 (subject particle) 3. 이렇게 하는 건: 건 is shorten from 거는, 거 + 는 (topic making particle) So, 1 and 2 are just similar because both use or drop Subject making particle. But 3 uses topic making particle for comparison or contrast. You use it when you want to compare something to another thing. So, you can say 3 when you want to suggest doing something compared to another possible options already being talked.