Birthday Gift Ideas - 3 Yr Old Twin Boys and 5 Yr Old Brother

The older child, will probably want and should get, a separate gift. He is older. AND, he probably does not like being all "lumped in together" by default, with his Twin siblings.
He is 5 now. They have different & separate, interests.
There is a big difference in development, between a 3 year old and a 5 year old.

Get a 'separate but the same type gift" for the Twins.
And then a separate gift, entirely, for the 5 year old.
And this will also then, curtail issues of "sharing" the same gift and children this age, not wanting too... and wanting their 'own' toy/gift.
So EACH child, will have something to open, and for it to be 'theirs.'

It would be great if you could give 3 separate gifts as it may be more challenging to find 1 big gift for all to share, but perhaps gifts that can use together. Like puzzles, games, car ramps and cars, crafts like play doh,a video etc. My twins just turned 5 last week and they have been playing the most with a car ramp that one of them got. I had gotten 2 ramps, but due to space, I returned one.

I think if mom is having a combined party it's ok to do a combined gift
It's summer so I'm thinking water toys. - like maybe a slip n slide along with 3 little water guns?
Or a seperate but the same gift of 3 big water soaker water guns.
Maybe a blowup pool for the 2 little ones and a super soaker for the big boy.

Definitely give separate gifts to the kids - they are 3 separate kids. If you know them well, different gits are ok, if you don't similar items are good to minimize fighting.

Mom of twins piping in: Separate but similar gifts for the twins and something different for the older brother. Getting 2 identical things for the twins can lead to an argument (that's mine, no it's mine) and since the 5 yr old is the big guy in the family he shouldn't get what the little guys get.

my now-9YO twin daughters have had a mix of separate similar gifts and shared gifts (craft kits, board games, etc.) and haven been okay with both. I agree with SH that the 5 year old should get a separate gift and would lean towards getting similar-but-separate gifts for the twins instead of a shared gift