I agree that it's important to encourage the correct grip, but it is a little too soon now for your dd.
She will find it easier with some tools than others - use a variety, eg big chunky pencils and paintbrushes in addition to smaller crayons etc. It may be worth investing in some rubber grips to go over pencils at some stage, these make it much easier for small hands.
I wouldn't push it, three is young, but see if you can do occasional short sessions 'correctly' then build on that. Don't put her off by insisting, as she will find it tiring at first. Could you maybe make a special time when you practice correctly - or using a special book for colouring it, or when 'writing' things with mummy, so that it doesnt affect her everyday creative sessions? If you do this for short daily periods she will develop better muscle control - just a couple of minutes at a time at first.
Hope this helps!