All three are abbreviations for ‘per centum’. The ‘per cent’ form is always written without a full stop and never in italics. Although traditionally written as two words in British English, the ‘percent’ form is gaining ground and occurs in 10% of all instances in the British National Corpus database. The standard American form is now ‘percent’ and South African English seems to follow this pattern.
The numbers accompanying ‘percent’ are normally printed as figures, such as 10,4 percent. Exceptions are made when round numbers or approximations are used, such as ‘twenty percent’. Words are preferred to figures at the start of a sentence, such as ‘Thirty-two percent …
The % sign is now freely used in most non-fiction writing. It is written immediately following the figures, with no space in between.