Ancestry has this 1891 census entry:
Name: William Clucas
Age: 3
Estimated Birth Year: abt 1888
Relation: Inmate
Gender: Male
Where born: Douglas, Isle of Man
Civil parish: Conchan
Ecclesiastical parish: Conchan
County/Island: Isle of Man
Registration district: Isle of Man
Sub-registration district: E
Institution: The Isle of Man Industrial Home for Orphan and Destitute Children
Piece: 4686
Folio: 122
Page Number: 4
Household Members included:
William Clucas 3
Charles Edward Clucas 14
Richard Kuisman Clucas 12
Robert Clucas 5
William Clucas 3

These last two boys look as though they were entered together, with dittos under the surname and place of birth (Douglas for both).
The older two boys were born Castletown (so unlikely to be brothers of Robert & William).
The first William Clucas aged 3 was b.Douglas.

Robert Clucas aged 5 could have been: Robert CLUCAS, b.21 June 1885, Douglas, father Henry CLUCAS, mother Margaret KELLY.
- Which doesn't help because I can't find a likely brother William.

The first William Clucas aged 3 could have been: William CLUCAS, born 18 Nov. 1887, Douglas, father Not given (i.e. illegitimate), mother Alice CLUCAS/BEATTIE. [BL index]

I thought this census entry could be a possibility because a child brought up from a young age in a children's home might not remember accurate details of his birth, or choose not to use them.

Sue