Even if they stamp 3 months (or 1 month) when she enters, she can apply for an extension and get it easily. As long as she gets in, I don't think it really matters how much time she has. The only difference is how soon she needs to apply for an extension.
Well, it is Some risk, but it also depends on some factors, such as if your wife is from a visa exempt country, she is less likely to be turned away at the border. Also, you could buy a fully refundable "return" ticket and refund it after you enter Canada.
Of course, at the border, she will not say she is moving to Canada. She should not bring too much luggage (a couple of suitcases is fine, but not household goods and furniture) and she should bring proof of having applied for PR. She should make it clear that she understands that she is a visitor and is able and willing to leave when she has to. Proof of having applied for PR is good because that shows she knows the rules and wants to follow the rules.
The thing is, whether you arrive in Canada then file the application, or whether you file the application and then come to Canada - doesn't make much of a difference at all. At least showing proof of PR application will show that you and your wife have made concrete plans for the future. (Instead of a couple who might enter Canada not having fully thought out PR and end up overstaying or something.)