My wife and I separated on October 1st 2014 which is when I moved out and rented an apartment. We have 3 children/dependants. We lived together for the first 9 months of the year. Support payments did not start until March 1st, 2015. We are only now finalizing a separation agreement. Who claims the children? My ex is under the impression that she claims them forever, and is entitled to child tax benefits and such. Our custody is and will always be joint and equal. Thank you in advance.
It's possible that a court ruling might stipulate the claimant in some conditions, but I don't have any knowledge in that area.
CRA says that in absence of agreement, neither parent can claim. This is a way of saying that they do not arbitrate disputes of child custody or tax credits for children.
In adition to what TonyC1 said (and having been in the situation), I would add that it is relatively common for parents in shared custody to divide the claims between them. Of course this is easier with an even number of children (as was my case); in your case, you can either alternate claiming the third child from year to year, or have the parent who will get the greater tax benefit make the claim, and compensate the other parent appropriately.
Again, though, all this depends on reaching an agreement, in the absence of which you both lose out. Note that if you both claim the same credit(s) for the same child, not only are both claims likely to be denied, but you risk penalties and possibly legal consequences.
The CRA has been generally slow in recognizing the reality of shared custody - it is only a few years ago that the CCTB was revised so that parents could split the benefit on an ongoing basis. It would be nice for families with shared custody if various credits could be split as well, and this might be a good year to mention that to your Member of Parliament.