  Context. Stellar coronal mass ejections (CMEs) are the primary driver of the
exoplanetary space weather and they could affect the habitability of
exoplanets. However, detections of possible stellar CME signatures are
extremely rare. Aims. This work aims to detect stellar CMEs from time-domain
spectra observed through the LAMOST Medium-Resolution Spectroscopic Survey
(LAMOST-MRS). Our sample includes 1,379,408 LAMOST-MRS spectra of 226,194
late-type main-sequence stars ($\rm T_{eff} < 6000$ K, $\rm log [g/(cm\
s^{-2})] > 4.0$). Methods. We first identified stellar CME candidates by
examining the asymmetries of H$\alpha$ line profiles, and then performed double
Gaussian fitting for H$\alpha$ contrast profiles (differences between the CME
spectra and reference spectra) of the CME candidates to analyze the temporal
variation of the asymmetric components. Results. Three stellar CME candidates
were detected on three M dwarfs. The H$\alpha$ and Mg I triplet lines (at
5168.94 {\AA}, 5174.13 {\AA}, 5185.10 {\AA}) of candidate 1 all exhibit a
blue-wing enhancement, and the corresponding Doppler shift of this enhancement
shows a gradually increasing trend. The H$\alpha$ line also shows an obvious
blue-wing enhancement in candidate 2. In candidate 3, the H$\alpha$ line shows
an obvious red-wing enhancement, and the corresponding projected maximum
velocity exceeds the surface escape velocity of the host star. The lower limit
of the CME mass was estimated to be $\sim$$8 \times 10^{17}$ g to $4 \times
10^{18}$ g for these three candidates.
