2 Samuel 24:13
13 So Gad came to David, and told him, and said unto him, Shall seven years of famine come unto thee in thy land? or wilt thou flee three months before thine enemies, while they pursue thee? or that there be three days' pestilence in thy land? now advise, and see what answer I shall return to him that sent me.
1 Chronicles 21:12
12 Either three years' famine; or three months to be destroyed before thy foes, while that the sword of thine enemies overtaketh thee; or else three days the sword of the Lord, even the pestilence, in the land, and the angel of the Lord destroying throughout all the coasts of Israel. Now therefore advise thyself what word I shall bring again to him that sent me.
This is often included in a list of numerical discrepancies between Samuel and Chronicles.
So, did Gad tell David that there would be 3 years of famine, or 7 years?
There are a number of potential solutions here.
One option is that the 7 year famine was specifically to "thee and thy land", i.e. the realm that David reigned over specifically.
The 3 year famine, then, which is given without any qualifying remarks, would apply to a presumably greater area extending to outside of his land.
If surrounding areas also experience famine for 3 years, those 3 years would be the worst bit. After that they would be able to get some relief by importing food.
Another potential solution is that the 2 Samuel account is retroactively adding to the year tally the 4 years of famine that they had already experienced up to that point.
Three chapters earlier, we are told:
2 Samuel 21:1
1 Then there was a famine in the days of David three years, year after year; and David enquired of the Lord. And the Lord answered, It is for Saul, and for his bloody house, because he slew the Gibeonites.
From this point, we are never told that the famine lets up, and it's at least another 9 and a half months until we arrive to chapter 24 because it took that long to number the people:
2 Samuel 24:8
8 So when they had gone through all the land, they came to Jerusalem at the end of nine months and twenty days.
Therefore the author of Samuel, in recounting the events, included the time already spent in famine in his paraphrase of what Gad told David, leaving the potential that Gad could have explicitly mentioned this smaller year count in reference to the total of 7 years, which he also may have mentioned.
Some add a third option, that these could have been multiple visits before/after David asked for mercy, which I suppose is possible, but it's purely speculative.
I find either of the two solutions I explicitly covered more likely, though. It's likely Gad said both 3 years and 7 years at some point while speaking to David about the judgement.