Oh, silly me! I read .....

three turnpoints which are all 10km away from each other.

Which implies 3 turnpoints in a tight little isoscelese triangle, he didn't
say 2 turnpoints 10km apart from each other and another 100km away from
either of those two points!

The format would work as a 3 TP declared free distance but would not
constitute an FAI triangle.

If you made an isoscelese with each tp 100km apart, one being the start
finish and the triangle centred on the home airfield you could do a 300km
FAI triangle and only get 66km from home. You'd have to fly 432km to acheive
the task though. Home, start at one apex, fly the triangle and then home.

Pull the same trick with the 3 tp's a mere 116 km apart , do the start
finish at the home airfield and you've got your 500 km diamond without
getting more than 76.5 km from home and you only fly the 500 km. What's more
you might be able to claim both diamonds off this task.

But why not try doing it the real way?
300km - Straight out
500 km - O/R or at least a full FAI triangle with home as one apex.
Thermic diamond height - can be done in Namibia

:-)

Ian