Difference between "voters", "electorates" and "constituents"

"Voters" are the people who vote (or more generally, those who are entitled to vote, whether they do so or not).

"Electorate" is usually a mass noun meaning "the collection of all voters". The use of the plural in your quotation I find rather strange and can only interpret it as "the collections of voters in different countries or polities" - its plural form occurring only because this is a general text talking about different countries.

"Constituents" are the people represented by a politician. I would use it to mean all those represented, whether they are voters or not, but there is room for some argument there. In most places "constituents" are a geographically defined group, but they could be, for example, the members of a profession if some body were organised to have representatives of professions.

They are different.

Constituents are people who live in a constituency. They don't necessarily have the right to vote, but are still represented by the elected person.

The electorate is the group of people who have the right to vote. It's important to note the context this is used in, as it could be the electorate of a whole country or the electorate of a local constituency.

Voters are people who vote. Although there are lots of people who have the right to vote, not all of them exercise that right, so not everyone in an electorate is necessarily a voter.

A word of caution, though: quite often, "voters" is used to mean everyone that can vote (as opposed to everyone who does vote) - but that's unlikely in a formal text about politics like the one you referred to in your question.

Here's my understanding:

A voter is simply an individual person who votes, or potentially votes.

An electorate is a defined geographic area that votes for the outcome of a single seat, or a set of seats. Electorate can also be used to refer to the collection of voters within that area.

A sitting parliamentarian's constituents are the voters within the electorate represented by him/her.

The three words have subtly different meanings, but in the paragraph you posted those distinctions don't seem to matter much at all. In this particular case you won't lose much meaning by reading them all as equivalents.