So...assuming that you're using two independent motors for the wheels-any reason not to create a Tadpole-style trike, with a free-flowing fixed rear wheel, then use the controllers on the two forward wheels in order to steer? A decent amount of cargo could still be carried, and it would be far safer. Admittedly you would then have to take Ackerman angles into account to avoid wheel scrub, but turning and stopping would be far less dangerous...
For an introductory treatise on steering geometry, I recommend the Link below, it will explain why so many on the Forum are worried for you:
Design considerations for three-wheeled on-road vehicles.
And a quick design summary of Delta vs Tadpole:
Having said that, the same designer has-on his own Designs page-several Delta trikes that worked well. Note however, that the single-wheel steer designs are both wide-stance and low-stance, while Tuk-tuks are rather the opposite. If your heart is set on a single-steer Delta (and there are some advantages to its mechanical simplicity) I would recommend:
1) Wide, low stance to prevent overspill upon turning/braking and to keep all weight firmly within the 'magic triangle'.
2) More weight towards the rear (same reason).
3) Really good back brakes and a weaker front brake (same reason).
Best of luck!