We study the non-classical properties of the single-mode electro-magnetic field resulting from the interaction of a \Xi-type three-level atom initially in the coherent state, such as squeezing properties and sub-Poisson statistics. We show that if there are more photons in the cavity, the squeezing will appear earlier and be stronger under the same state, but the sub-Poisson statistics will be weaker, while sub-Poisson statistics and squeezing are more pronounced after the selective atomic measurement.