  The associated production of a single-top with opposite-sign same-flavor
(OSSF) di-leptons, $pp \to t \ell^+ \ell^-$ and $ pp \to t \ell^+ \ell^- + j$
($j=$light jet), can lead to striking tri-lepton $pp \to \ell^\prime \ell^+
\ell^- + X$ and di-lepton $pp \to \ell^+ \ell^- + j_b + X$ ($j_b=b$-jet) events
at the LHC, after the top decays. Although these rather generic multi-lepton
signals are flavor-blind, they can be generated by new 4-Fermi flavor changing
(FC) $u_i t \ell \ell$ scalar, vector and tensor interactions ($u_i \in u,c$),
which we study in this paper; we match the FC $u_i t \ell \ell$ 4-Fermi terms
to the SMEFT operators and also to different types of FC underlying heavy
physics. The main backgrounds to these di- and tri-lepton signals arise from $t
\bar t$, $Z$+jets and $VV$ ($V=W,Z$) production, but they can be essentially
eliminated with a sufficiently high invariant mass selection on the OSSF
di-leptons, $m_{\ell^+ \ell^-}^{\tt min}(OSSF) > 1$ TeV; the use of $b$-tagging
as an additional selection in the di-lepton final state case also proves very
useful. We find, for example, that the expected 95\% CL bounds on the scale of
a tensor(vector) $u t \mu \mu$ interaction, with the current $\sim 140$
fb$^{-1}$ of LHC data, are $\Lambda < 5(3.2) $ TeV or $\Lambda < 4.1(2.7)$ TeV,
if analyzed via the di-muon $\mu^+ \mu^- + j_b$ signal or the $e \mu^+ \mu^-$
tri-lepton one, respectively. The expected reach at the HL-LHC with 3000
fb$^{-1}$ of data is $\Lambda < 7.1(4.7)$ TeV and $\Lambda < 2.4(1.5)$ TeV for
the corresponding $u t \mu \mu$ and $c t \mu \mu$ operators. We also study the
potential sensitivity at future 27 TeV and 100 TeV high-energy LHC successors
and also discuss the possible implications of this class of FC 4-Fermi
effective interactions on lepton non-universality tests at the LHC.
