I want to show three decimals for my fluid flow sensor

[sterretje]

Your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advice on) your project See About the IDE 1.x category.

I think that Serial.print() - Arduino Reference will give you the answer.

An optional second parameter specifies the base (format) to use; permitted values are BIN(binary, or base 2), OCT(octal, or base 8), DEC(decimal, or base 10), HEX(hexadecimal, or base 16). For floating point numbers, this parameter specifies the number of decimal places to use.
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• Serial.print(1.23456, 2) gives "1.23"
• Serial.print(1.23456, 4) gives "1.2346"