blossom drop.

i noticed a couple of flowers falling off not loads just a couples off 2 or 3 plants is it normal for this to happen i have got fruit on them they look ok.
my first yr with a greenhouse so loads to learn what is a truss i thought a truss was a bunch of the yellow flowers.
im watering once a day and twice when its hot how much water should i give a 5ft plant there in the black tomato pots.

 

Probably just a couple that had not been pollinated.
A truss is as your say a group of yellow flowers on your tomato plants.
Botanically a compact cluster of flowers at the end of a stalk.
Twice a day for maturing tomato plants is not excessive. Regarding quantity, that can only come by experience. If the pots and/or bed are freely draining it would be difficult to overwater.
Always apply light shading to a glasshouse in summer.