Upvote:0
A multiple-entry visa means that after the first trip, which has to be in accordance with your application, you can make subsequent trips up to the limit of the duration and validity.
This makes a lot of sense when you think about it. They're afraid of illegal immigrants. If you have been to Spain and did not overstay, this makes it likely that you will not overstay in Italy, either.
There are some things to keep in mind when you enter the Schengen area:
Basically, prepare your trip as if you had to make a new visa application, and have the documents ready when you arrive.
Upvote:2
The answer to your question may be found in Annex VII of the Schengen Visa Code. Annex VII describes the visa sticker in detail. About "duration of stay" it says:
This heading indicates the number of days during which the visa holder may stay in the territory for which the visa is valid. This stay may be continuous or, depending on the number of days authorised, spread over several periods between the dates mentioned under 2, bearing in mind the number of entries authorised under 3.
(Item 2 concerns the visa's period of validity; item 3 indicates whether the visa is valid for one entry, two entries, or multiple entries.)
Since you can "spread" these days "over several periods," we can conclude that the three days you've spent in Spain have consumed only three of the 45 days you've been granted, leaving 42 days for you to spend in subsequent visits during the visa's period of validity. A ten-day trip in August is well within the terms of your existing visa.
(This assumes that your visa is valid either for all "Schengen states" or for a subset of states that includes Italy. It should also be noted that your stay was three days if you arrived on the 30th of March and left on the 1st of April. The calculation will be done according to the dates stamped in your passport; if you left after midnight and the departure stamp shows 2 April, then your stay was actually four days in duration.)