Amotivational symptoms are observed in schizophrenia (SCZ), bipolar disorder (BD), and major depressive disorder (MDD). Effort-cost computation may be a potential contributor to amotivation transdiagnostically. This study examined effort-cost computation in these three diagnostic groups. This study recruited 141 outpatients (49 SCZ, 52 non-psychotic BD, and 40 non-psychotic MDD) and 57 healthy controls (HCs). We administered the Effort-Expenditure for Reward Task (EEfRT), which manipulated different levels of reward magnitude and probability relating to a high and low physical effort task. There were significant interactions between group and reward magnitude, group and reward probability, and group and expected value on the percentage of high-effort choices. SCZ, BD, and MDD patients made comparably fewer high-effort choices than HCs in the high-reward magnitude, high-reward probability, and high-expected-value conditions. Self-reported amotivation did not correlate with decision-making on the EEfRT. Our findings suggest that reduced effort expenditure for reward is a transdiagnostic phenotype in SCZ, BD, and MDD.