I cannot understand if carbonyl adjacent to alkene stabilises or destabilises it. Generally, we say that more substituted alkene is stable. In those cases attached groups increase electron density of the alkene. In the case of carbonyl, it decreases electron density of the alkene so it should destabilize the alkene.
But I read about the stability order of the above compounds to be 1>2>3. The reason being that the first is conjungated and second's enol is conjungated.
So, how can both increasing and decreasing the electron density of alkene stabilise it?