Just like the red and orange camps, StarHub is giving you the option to add more data to your plan. The surprise comes in the form of how much data is given. Singtel doubles the amount of data pegged to the plan while M1 adds a fixed amount to each tier.
In StarHub's case, you pay S$6 for 3GB of data, S$3 if you sign up by 31 March 2016.
Warning, incoming mathematics lesson.
A quick glance across all the plans (as above) gives you an idea of the new amount of data you're getting for each plan. For sure, S$3 for 3GB is exceptionally cheap, considering that excess charges at S$10.70 per GB. Even after the promotional rate is no longer valid, at S$6 for 3GB, you're paying only S$2 per GB.
Taking the promotional rate of S$3 out of the equation, this is how you should compare the StarHub data add-on to Singtel and M1.
In comparison to Singtel and M1, the extra 3GB is definitely worth the money on the lower-tier plans. With the 4G 3 plan, you'll pay S$45.90 for 6GB of data (S$48.90 after the 31 March promotion). In Singtel's case, for a similar cost of S$48.80, you only get a total of 4GB of data on the Combo 2 plan while M1's i-Lite+ or Lite+ plans grant you 5GB of data.
But as you go up the tiers, both M1 and StarHub are neck to neck for the 4G 4 and i-Reg or Reg plan, both giving 7GB of data. Singtel is short by 1GB for a similar price around S$68.80.
Towards the higher end plans, StarHub's flat addition of 3GB starts to show some cracks. Both Singtel and M1 offer more data at 12GB and 13GB respectively while StarHub gives you only 9GB for a similar price.
tl;dr - if you are sticking to the lower end price plans, StarHub's 3GB addition, even at S$6 for 3GB, gives you the most value for money. Heck, if you sign up by 31 March, it's at least S$3 cheaper than what you're paying to Singtel and M1.
Just be aware that it's not that attractive as you go towards the higher tier plans.