If I check something 3 times, how may I refer to them using a gerund? Are these sentences ok?
a. All 3 checkings found some problems.
b. All 3 checking found some problems.
And how about other verbs like eating, walking, or beating? Can we say eatings, walkings, or beatings?
Are there any rules when we can put "s" to a gerund and when we cannot?
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.
