We find an explicit representation of the evolution of \({ t \mapsto \gamma_t = \{z (\zeta, t), \zeta \in \mathbb{C}, |\zeta| = 1 \} }\) of the contour \({ \gamma_t = \partial \omega_t }\) of fluid spots \({\omega_t = \{z (\zeta, t), |\zeta| 0}\) or \({t 0}\)) or a source (\({t < 0}\)) localized at point \({z(0, t)}\) described by trinomials $$z(\zeta, t) = a_1(t)\zeta + a_N(t) \zeta^N + a_M(t) \zeta^M,{\rm where} \quad M = 2N - 1,\quad{\rm and\,integer} \quad N \ge 2,$$ for the classical formulation of the problem when \({\omega_t }\) is within \({ \gamma_t }\) (inner Hele–Shaw problem), or by $$z(\zeta, t) = a_{-1}(t)\zeta^{-1} + a_N(t) \zeta^N + a_M(t)\zeta^M, {\rm where} \quad M = 2N + 1,\quad{\rm and\,integer} \quad N \ge 1,$$ for the outer Hele–Shaw problem when \({ \omega_t }\) is outside of \({\gamma_t}\). We obtained a sufficient condition for univalence of real trinomials, improving a result found by Ruscheweyh and Wirths (Ann Pol Math. 28:341–355, 1973). A sufficient condition is also found for functions used in the outer problem.