Can a employer/union deny wage increases while on benefits with LTD? I have remained working for the 3 years I've been on LTD.

I'm not sure I understand your question. If you mean your regular employment wage, you would need to speak with your union representatives about this matter.
If you're referring to your disability benefit amount, your insurer generally only has the obligation to pay your LTD benefits based on the income you were earning when you started your LTD claim, unless your policy has a Cost of Living Increase provision.