3 yr old will enjoy it but won't give the other 3 much of a chance to try stuff out perhaps, not the same concentration span .
We were there last week, the new Alice through the looking glass exhibits on the top floor are good. TBH the 3 older ones will love it all.
The "Flame on" show is really good (3yr old might be a bit scared as the lights go down and big bangs), go to the earliest show as they get busy later on,
There are work shops and shows all day, if you want to do the work shops put hte kids names down for the afternoon ones and go to the morning shows as you will have to queue for them.
Bubble work shop was a bit tame even for the 6 yr 5 yr olds, you could do more adventurous stuff in the garden.
Photography work shop will be to complicated for 3 yr old and the 5/6 yr olds will need help so no good if you are the only adult.
No indoor areas for picnicing, cafe is OK, hot food turns into cold food by the time you get past the till, there is a pick and mix kids meal bag with cheese rolls and stuff. plenty choice of sandwiches, crisps etc.
I would also suggest the transport museum and the Kelvingrove being free is always a bonus. Very close so one in the morning and one in the afternoon, if it is dry picnic in the park beside the Kelvingrove.
I would save the money and just take the older 2 to the science museum or at least when you don't have another kid as well. I hope you will have help, so many things to see at any of these places, I only have 2 but they go of in opposite directions. Have fun .