You have a 3-month notice period. How long, realistically, will it take you to find a new job? If the answer is less than 3 months, then what you must do is give notice now. If it is longer, then brush up your resume, begin the job search, and then give notice.
As acknowledged in other answers, you've already talked to your manager and basically gotten nowhere as a result. The fact that you're being lumped into a company-wide initiative is means that no consideration is given to your skills, or the market rate for them, or to your bus-factor. In other words, you're just a faceless number, and you're going to get the same 1.5% (wild guess) that everyone else does.
If you give notice now, particularly given your low bus-factor, that gives your manager leverage to make a stink with HR and give you a (more) immediate counter-offer to stay. This counter-offer would likely be far higher than the part-of-a-company-wide-initiative raise you would have gotten otherwise.
And if you don't get one, well... then you clearly have your answer about whether they plan to ever pay you what you're worth, and you have a jump on working out your notice period so you can get to a job that pays you what you need.
And to just re-emphasize a point made in other answers, the best raises come with new jobs - whether that's an in-company promotion, or a move to a new job. So even with a counter-offer, you're probably still not going to get what you're worth on the market.