How would I concatenate 'inputObject' to match Enum.KeyCode?

This isn't too simple of a problem as it requires some logical thinking. When you face something like this, break it into multiple smaller components -- that's the first step into modular programming. The way you've thrown everything into one "anonymous" function is not a good practice and will really hurt in the long term as it is very messy.

Here's how I'd approach this issue, without any extra modules. An improvement would be to get rid of the global variables and treat the sequence as one object that handles its own logic. Its creator would then make binds for each step in the sequence.

local Players = game:GetService("Players")
local UIS = game:GetService("UserInputService")

local localPlayer = Players.LocalPlayer
local mouse = localPlayer:GetMouse()

--

local SeqState = {
    none = 0,
    first = 1,
    second = 2,
    third = 3,
}

local inputToSeqState = {
    [{Enum.KeyCode.E}] = SeqState.first,
    [{Enum.KeyCode.One, Enum.KeyCode.Two, Enum.KeyCode.Three}] = SeqState.second,
    [{Enum.UserInputType.MouseButton2}] = SeqState.third,
}

--

local carrierSlots = {}
local lastSequenceState = SeqState.none
local lastChosenNumber = nil

--

    --  Carrier placement logic
local function placeCarrier(carrier, pos)
    carrier.CFrame = CFrame.new(pos)
    carrier.Parent = workspace
end

local function getNumberFromKeyCode(k)
    --  There must be a simpler way
    local e = Enum.KeyCode
    return  (k == e.One or k == e.KeypadOne) and 1 or
            (k == e.Two or k == e.KeypadTwo) and 2 or
            (k == e.Three or k == e.KeypadThree) and 3
end

local seqFunctions = {
    [SeqState.second] = function(inputObject)
        --  This shouldn't be done in the real world as it is a bad practice
        lastChosenNumber = getNumberFromKeyCode(inputObject.KeyCode)
    end,

    [SeqState.third] = function(inputObject)
        local n = lastChosenNumber
        placeCarrier(carrierSlots[n], mouse.Hit.p)
    end,
}

--

local function isInArray(array, val)
    for i = 1, #array do
        if array[i] == val then
            return true
        end
    end
    return false
end

local function getSeqState(inputObject)
    for validInput, seqState in next, inputToSeqState do
        if isInArray(validInput, inputObject.KeyCode) or isInArray(validInput, inputObject.UserInputType) then
            return seqState
        end
    end
end

local function stepSequence(seqState, inputObject)
    if seqFunctions[seqState] then
        seqFunctions[seqState](inputObject)
    end
end

local function isNextStep(last, new)
    return new == last + 1
end

local function processInputBegan(inputObject, processed)
    if not processed then
        local seqState = getSeqState(inputObject)
        if seqState and isNextStep(lastSequenceState, seqState) then
            lastSequenceState = seqState
            stepSequence(seqState, inputObject)
        end
    end
end

local function processInputEnded(inputObject, processed)
    if not processed then
        local seqState = getSeqState(inputObject)
        if seqState then
            if seqState <= lastSequenceState then
                lastSequenceState = math.clamp(seqState - 1, SeqState.none, math.huge)
            end
        end
    end
end

--  Event connections

UIS.InputBegan:Connect(processInputBegan)
UIS.InputEnded:Connect(processInputEnded)

--  Demonstration phase

local function createPart(props)
    local p = Instance.new("Part")
    for prop, val in next, props do
        p[prop] = val
    end
    return p
end

table.insert(carrierSlots, createPart{BrickColor = BrickColor.Red(), Anchored = true})
table.insert(carrierSlots, createPart{BrickColor = BrickColor.Green(), Anchored = true})
table.insert(carrierSlots, createPart{BrickColor = BrickColor.Blue(), Anchored = true})

It doesn't require anything extra, you can just drop it into a LocalScript inside StarterPlayerScripts. I'd still suggest to experiment with it and implement it on your own, for the sake of experience.