The effect on the election result, in Italy like in most countries is the same in all three cases: the vote will be discarted and the result will be determined by the valid votes. It is like saying "whatever the other voters want".
The same effect is obtained by voting on a little party that obtains less than the required threshold (for example 4% for the Italian Chamber of Deputies in the case of parties running outside of a coalition).
The difference of course is in the message:
If you abstain it looks like you do not care.
A blank or null vote seems to indicate that there is no party you really like, so it doesn't matter and you can as well accept what the others vote for. Some people might hope that if there are many blank votes this will create scandal and somehow improve the situation. A null vote can also indicate that the voter made a mistake.
Votes for small parties could be from voters hoping the party would pass the threshold. Some people have however suggested to purposefully vote on parties with no chance of going to the parliament, with the aim of showing how unrepresentative the parliament is.