It's fairly unlikely that births wouldn't have been registered after 1875.
The most common reason for a failure to find a birth is because you;re looking under the wrong name - or, as you would see it, the birth was registered unde the wrong name.
People with unusual surnames - and they don't get much more unusual - sometimes changed them, and just to confuse us, they subsequently changed them back again.
Anyway, if the dates for birth aren't given in the baptism entries your first step is to find their 1939 register entries and/or their post 1969 death entries, so that you know precisely when they were born.
Something else to consider is that the family weren't in England & Wales when those three chilldren were born. Birthplaces aren't always shown correctly in the census.]
Finally, a reminder that this forum is a privilege for LostCousins members, and if you are researching on behalf of someone else they need to join LostCousins, even though you're the one doing the research. Otherwise you would be depriving their 'lost cousins' of the opportunity to connect.
You can, of course, use one of your own email addresses for their account.