Personally, I think after three years (with a child in that mix) it makes sense to not be open to poly but I wonder if this might change. My partner and I were solo with each other for over 10 years before we were ready and he did a lot of things to accomidate my sexual needs, so perhaps there is space for a less hetero relationship within your existing bond.
I don't think that three years is too early to open things up...if indeed that's what the quoted poster was saying. I think its completely dependent on those involved. We opened our marriage up after only one year and its been several years now. Things get better each day! We had one child when we opened the marriage up, got pregnant and had our second while open and continue to be blissfully happy. For us, our outside relationships and flings...yes some are just flings....make our own bond stronger.
There are several options for opening up your marriage - relationships aren't necessarily required, if that's something you would be comfortable with.