MessageLevel in QGIS3

The proper syntax is like below:

# First import Qgis class
from qgis.core import Qgis

...

def something():
    # Then you have to use like 'Qgis.MessageLevel' (Qgis.Info as an example)
    iface.messageBar().pushMessage(text, level=Qgis.Info)

For more information, please review Communicating with the user and QGIS API Documentation.

You can make a util function like this,

from qgis.core import Qgis as QGis

def showUserMessage(before, text="", level=QGis.Info, duration=3):
        ''' Show user info/warning/error messages '''
        iface.messageBar().popWidget()
        iface.messageBar().pushMessage(before, text, level=level, duration=duration)
        return

showUserMessage( "Info : ", "Info Sample")
showUserMessage( "Warning :", "Warning Sample",QGis.Warning)
showUserMessage( "Critical : ", "Critical Sample", QGis.Critical)