Another way to view the two versions. First, contract "it" with "is" (how you'd probably hear these in practice ( < another economy of speech)):
(a) It's three years since my father died.
(b) It's been three years since my father died.
Next, consider that (a) simply looks like it is using simple present. What (a) vs. (b) might be showing is an economy of speech--that you don't have to say what will be understood anyway:
(a) It's been three years since my father died.
(b) It's been three years since my father died.
From this angle, (a) is simply an abbreviated, more economical way to say (b). It appears to be: simple present + period of time + since, but on a deeper level it is really (a shortened) present perfect.
Is it correct to look at these two sentences this way? Given the grammar book that I linked to, above, probably not. But looking at (a) in this way may help a student understand why simple present can be okay here.