The tenants are equally responsible for the ordinary cleanliness of the unit. A mutually-agreed upon cleaning schedule may be helpful to divide the workload between the three of them. You may hire a cleaning service, but it should be specified in the tenancy agreement (especially if the cleaners are going to be entering the rental unit at a designated time every month).
If a tenant is not fulfilling his or her cleanliness obligations under the Act, you may send a reminder letter letting your tenant know that they must keep the unit clean, a copy of section 33 of the Residential Tenancies Act can also be attached (which indicates the tenant is responsible for ordinary cleanliness of the rental unit). If the tenant still fails to comply then a N5 notice may be served.