Not every word ending in "ing" is a gerund, even when functioning as a noun. The existence of a plural is a good sign that the word isn't a gerund, but a full noun:
Three buildings are located on the site (Building is a noun)
Building a house is a lot of work (building is a gerund)
The example in comments is similar. A "newspaper cutting" is a noun, not a gerund.
Gerunds can function in place of a noun, and they behave somewhat like uncountable nouns. They do not have a plural form. If you see a plural, that suggests that the gerund has been upgraded to a noun.
In you specific example, we don't use "checking" as a noun, perhaps because it is not needed. The noun "check" is a countable noun that can be used in this context.
All three checks found some problems
There are other differences between gerunds and nouns. For example nouns get modified by adjectives; gerunds are modifided by adverbs.
Checking carefully is the last stage of the process.
A careful check is the last stage of the process.
So, in short, gerunds don't have plurals. But some nouns can look like gerunds.