Git methodology question

All,

I am trying to setup a "reliable" flow with git. I am sort of new to the tool and need some help from the experts. The question pertains to the way SHA1 tags can be back-tracked in parallel clones. Here is the scenario I have in mind:

In order of time: User1 – pulls from the origin; updates, modifies, verifies and commits into his clone. This will generate SHA1_1 User2 – pulls from the origin; updates, modifies, verifies and commits into his clone. This will generate SHA1_2 User3 – pulls from the origin; updates, modifies, verifies and commits into his clone. This will generate SHA1_3

In none of these cases, anyone is updating from each other’s clones

At the end of the week, the model builder needs to merge and verify all commits from user1, user2 and user3 before pushing into the origin master. 

The question is,
            Can the model builder use SHA1_3 tag only to get all the updates made by user1, user2 and user3; or all SHA1 tags are needed?

Thanks, Farshad.