You can safely read up to book 3 without spoiling the tv show, if that’s your priority. But it’s getting quite difficult to line up the timelines between the books and the series, since about the middle of season 3. I’m reading book 5 now, and there are spoilers for me in the current season. In other words, the writers of the series (sometimes Martin is among those writers) are placing some events from the very latest writings into season 4 – even though they still haven’t covered some major events from book 4. It’s a bit crazy.
So, just be aware that you might not be able to read beyond book 3 for, like, another year or so. But I’m going to bet that if you make it that far, you won’t be able to wait. ;)