I was trying Finn out with some observational drawing recently (using a soft toy- in hindsight too complicated for a three year old).
He started off drawing the monkey for a minute or so, but was quickly becoming frustrated that it wasn't looking like the object he was trying to draw, so he went off on his own little drawing adventure.
I try not to structure his drawing too much and keep it all very fun and light. Sometimes an object for observational drawing is good for a starting point if the child isn't sure what to draw. Any kind of drawing and mark making is a very good thing.