Where can I find reliable data for melting points of organic compounds?
You are absolutely right about suspecting the reliability of sources, based on their value spread. I think, I found what's going on with those internet (I assumed) sources. Yet, I know everybody agree with me that CRC handbook of chemistry and physics is The Bible for Physical Constants of Organic Compounds. Here is what I found on 2005 Internet Version of CRC handbook of chemistry and physics:

3-Methylpentane (CAS #: 96-14-0): physical form: liquid; Melting Point: $\pu{-162.90 °C}$; Boiling Point: $\pu{63.27 °C}$; and Density: $\pu{0.6598 g/cm^3}$ (pp. 3-398 of Ref.1) 

Perfluoro-3-methylpentane (CAS #: 865-71-4): physical form: liquid; Melting Point: $\pu{-115 °C}$; Boiling Point: $\pu{58.4 °C}$; and Density: not listed (pp. 3-458 of Ref.1) 

I think some reference sources may have mistaken with values of perfluoro-3-methylpentane as those of 3-methylpentane.

Nonetheless, evem PubChem listed two other values than $\pu{-162.90 °C}$ for melting point of 3-methylpentane (Ref.2): $\pu{-162.90 °C}$ from Lide, D.R., G.W.A. Milne (eds.). Handbook of Data on Organic Compounds. Volume I; 3rd ed.; CRC Press, Inc.: Boca Raton, FL. 1994., p. V4: 3865 (most trusted value since it is PEER REVIEWED - https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/r?dbs+hsdb:@term+@rn+@rel+96-14-0); $\pu{-118 °C}$ from http://www.ilo.org/dyn/icsc/showcard.display?p_version=2&amp;p_card_id=1263 (International Chemical Safety Cards (ICSC) are data sheets intended to provide essential safety and health information on chemicals in a clear and concise way); and third value of $\pu{-245 to -148 °F}$, basically, $\pu{-153.9 to -100 °C}$, from  OSHA Occupational Chemical DB (http://www.osha.gov/chemicaldata/chemResult.html?RecNo=764). I can only speculate that they all listed wrong values because they got hits for some other substituted 3-methylpentanes with search keyword of 3-methylpentane. For example, CRC Handbook of Chemistry and Physics listed $\pu{118.27 °C}$ for boiling point of 3-ethyl-3-methylpentane (CAS #: 1067-08-9).

Yet, only CRC Handbook of Chemistry and Physics consistently listed $\pu{-162.90 °C}$ as the melting point of 3-methylpentane since 1994. Thus, I'd take that value at any time.

References:


"Section 3: Physical Constants of Organic Compounds", in CRC Handbook of Chemistry and Physics, Internet Version 2005, David R. Lide, ed., http://www.hbcpnetbase.com, CRC Press, Boca Raton, FL, 2005 (pp. 3-398).
https://pubchem.ncbi.nlm.nih.gov/compound/3-methylpentane#section=Melting-Point