1)The ITR can be revised only before assessment of return. So, you can go for rectification of return.
2) ITR-3 is for the assessee who are going for presumptive taxation. and ITR4 is for the assessee who has to keep its books of account and are taxed at normal provison.
3) ITR6 is applicable for only companies. However, audit here referred is Tax audit. If you have gross receipt more than 2 crores or if you are declaring lower profit U/S44AD then you are liable to get your books of accounts audited.
4)Company are liable to get statutory audit under companies Act. As you have mentioned earllier, there were no income under company, you are not liable to get tax audit. However, you are liable to file ITR6.
5) ITR6 still apply for companies, because they are liable to get account maintained. and
If you are trading under proprietorship business you can go for 8% and file ITR 3.
Feel free to know if you require further clarifications