What is the legal difference between an apartment, unit or suite ?

It depends whether you are referring to a rental apartment or a condominium. Generally the word apartment is used for a rental however in our Real Estate MLS system we do use it as a description type meaning "Condo Apt vs a Townhouse". Unit is commonly used for condominiums rather than suites. If the unit was furnished then suite would be the more common terminology. The world of Condominiums can be confusing because there is so much for the general public to learn including the fact that there is a Condo Owners Association which is a registered non profit association representing Condo Owners Check into: www.COAontario.com
On the legal side, assuming you are referring to a condo; there is no difference they all fall under the Condo Act.

As far as I am aware there is no legal difference between them. Apartment and Suite are mostly used for high rise condominiums and unit can be used for many different types of properties.

I would agree with the above posts. Really depends on the building but there doesn't seem to be a legal difference. The same space can be addressed a unit, loft, suite or apartment.

Generally speaking, an apartment or suite are architectural terms describing a "Dwelling" in a multi residential or commercial building. Unit is a legal term that identifies a specific space. This could be an apartment or suite, or a lot in a condominium or strata development, such as "unit 23 in condo plan 2089765" a opposed to the civic address such as suite "106, 2593 Main Street". The unit would be the way it would be identified in the Land Titles Registry office but the suite or apartment number would be the way your mail would be delivered or visitors would find you.

I don't think there is a legal difference, these terms can be used to describe the same "unit"