Did he need to 3 stop? No.
However, RBR were running 2nd and 3rd, it's lap 25, two ways to get the win here. Have Webber maintain the 2 second gap, close up just before his 2nd (and final) stop and try and undercut. Is that going to work? Well, Lotus has better tyre life as far as we know, this is borne out in the first stint, so it might not work. Secondly, even if Webber has better tyre life than Grosjean, when it gets to the 2nd (final) stop window, Webber goes out of the 2-2.5 window and starts closing up, easily giving Lotus the warning they need to bring Grosjean in and cover him either on the same lap, or a lap earlier? So, yeah undercut might've worked, but it might not have.
Second option, split the strategies, goad Lotus into covering both to maintain track position by pitting very early for their 2nd and final stop. Defend everywhere, defend nothing! Worst case it all flops over, and you're in the exact same situation (2nd/3rd) at the end. As an added bonus, if you move Webber onto a 3 stopper, Vettel moves up into Webbers spot, giving you the option of Webbers previous undercut strategy if Lotus don't fall for it. For me, this is clearly a superior option.
If you make that decision you have to chose who is going to go onto the 3 stopper. No brainer really. Seb is already on fresher tyres, has shown better tyre management like every race ever, has a higher downforce setup which will just amplify that, and Mark has a higher top speed which in theory should give him an easier time overtaking. Not to mention he's already ahead, makes perfect sense to me.