How do I get players in a Region3 area?

The workspace has a function called FindPartsInRegion3, which takes a Region3 object to do as it name implies: find the parts in that Region3.

As you mentioned earlier, you have:

local a = Region3.new(Vector3.new(1, 1, 1), Vector3.new(10, 10, 10))

Now, what you need to do is make use of the FindPartsInRegion3 function of workspace to retrieve all the parts in a. For each part found, you need to determine if the part belongs to the character of a player or not. It can be done so like this:

local players = {}
local a = Region3.new(Vector3.new(1, 1, 1), Vector3.new(10, 10, 10))

for _, part in pairs(workspace:FindPartsInRegion3(a, nil, math.huge)) do
    local character = nil
    -- In the case the part is a limb
    if part.Parent:FindFirstChild("Humanoid") then 
        character = part.Parent
    -- In the case the part is part of an accessory
    elseif part.Parent.Parent:FindFirstChild("Humanoid") then
        character = part.Parent.Parent
    end

    if character then
        local player = game.Players:GetPlayerFromCharacter(character)
        if player and not players[player.Name] then
            players[player.Name] = player
        end
    end
end

Now, the players table will contain all the Players found in the specified Region3, with the key being the Player's name and the value being the Player object.