3 years back or ago?

Macmillan does not mark this sub-sense of back as informal or colloquial:

back [adverb (5)]: used for talking about a period of time in the past

Back in the ’70s, disco music was very popular.

She had a minor operation a few years back.

Things were different back then.

[bolding mine]

However, people on various other websites {eg massromantic at WordReference} consider 'three years back' less formal than 'three years ago', and I'd agree with them, especially for British usage:

"Yes, I think it ['a few years back'] is slightly more informal. I don't see it written often -- I usually just hear people say it."