12V to 24V 3A boost converter using LM2587
Take a look at this: -



This is from the data sheet and I'd say that it implies that one device would max-out if you tried to take more current than 1A. Two circuits would produce 2A at 24V using the technique shown in the other link so realistically you'd need three devices.

24 V at 3 A is 72 W and is probably asking too much of this type of device to provide half this power. You could still use three (tying the compensation pins together and shorting feedback to ground on two of them but it looks less attractive now.

I haven't looked into the details much but it seems that in boost mode it's better suited to provide higher output powers when using a larger input supply voltage - figure 53 shows a 36 V output at 2 A from a 24 Vnom supply for instance. Figure 54 is starting to hint at the device using a heatsink (48 V @ 1.5 A output from a 24 Vnom supply).

You just might get away with 24 V @1.5 A (3 A from 2 chips) if you used a heatsink but it'll be close!

I'd favour a more modern, higher frequency switcher from Linear Tech. They have a substantial range and an easy selection tool for finding devices that fit the bill. The LT3579 is about the best they offer with internal switching transistors so I think you are probably going to be looking at a chip with external transistors for switching.

However, the LTC3787 using external transistors and bi-phase drive for reduced ripple looks the beast for generating 24 V at 10 A - see figure 10.