Basic Thirdperson

local thirdperson = CreateConVar("3rdperson", "0", FCVAR_USERINFO)

local m_yaw = GetConVar("m_yaw"):GetFloat()
local m_pitch = GetConVar("m_pitch"):GetFloat()
local view_angle, view_origin = EyeAngles(), EyePos()

hook.Add("CalcView", "Cool 3rd Person", function(ply, origin, angles)
    if not thirdperson:GetBool() then return end

    -- Use a hull trace to prevent the camera clipping through the world
    view_origin = util.TraceHull({
        start = origin,
        endpos = origin - view_angle:Forward() * 100 + view_angle:Right() * 15,
        filter = ply,
        mask = MASK_SHOT,
        mins = Vector(-5, -5, -5),
        maxs = Vector(5, 5, 5),
    }).HitPos

    return {
        angles = view_angle, -- Set the camera view to the calculated angle
        origin = view_origin, -- Set the origin to be thirdperson
        drawviewer = true,
    }
end)

hook.Add("StartCommand", "Cool 3rd Person", function(ply, cmd)
    if not thirdperson:GetBool() then return end

    -- We need to calculate mouse movement ourselves since we need to set the view angles
    view_angle = view_angle + Angle(cmd:GetMouseY() * m_pitch, cmd:GetMouseX() * -m_yaw, 0)
    view_angle[1] = math.Clamp(view_angle[1], -89, 89)
    view_angle[2] = math.NormalizeAngle(view_angle[2])

    -- Movement will get wonky in any other MOVETYPE
    if ply:GetMoveType() == MOVETYPE_WALK then
        -- Run a trace from the 3rd person camera
        local view_trace = util.TraceLine({
            start = view_origin,
            endpos = view_origin + view_angle:Forward() * 1e4,
            filter = ply,
            mask = MASK_SHOT,
        }).HitPos

        -- Update view angles so we're facing where the camera trace hit
        cmd:SetViewAngles((view_trace - ply:GetShootPos()):Angle())

        -- Because we've meddled with the view angle we need to update the movement so it works as expected
        local move_fix = Vector(cmd:GetForwardMove(), cmd:GetSideMove(), 0)
        move_fix = (move_fix:Angle() + (cmd:GetViewAngles() - view_angle)):Forward() * move_fix:Length()

        cmd:SetForwardMove(move_fix[1])
        cmd:SetSideMove(move_fix[2])
    else
        cmd:SetViewAngles(view_angle)
    end
end)