Howdy!
That's not really how to set a Vector3 value. According to the Roblox Documentation on Vector3 Values, all you need to define is the X, Y, and Z value to set it to. You don't need to set where it goes to, as it is predefined by a format.
For example, if you wanted to change the size of a part in the workspace, you would use what I have below.
1 | local part = workspace.Part |
2 | part.Size = Vector 3. new( 5 , 20 , 100 ) |
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Be sure to check out the Roblox API Documentation as well for additional reference.