Can Aluminum Hydroxide (Al(OH)3) be heated to it's melting temperature before decomposing?
TL.DR: Aluminium oxide (sometimes known as alumina) is made by heating the aluminium hydroxide to a temperature of about 1100 - 1200°C. (Chemguide)

$$\ce{Al(OH)3 -&gt;[\Delta] Al2O3 + 3H2O}$$



As @andselisk said, the temperature Wikipedia mentions is aluminium hydroxide mixture in a fire retardant which might have other compounds. Decomposition of pure aluminium hydroxide has been studied in various papers and its decomposition reaction mechanism, temperature, reaction kinetics has been exhaustively studied.

A paper examines$\ce{^{[1]}}$ the decomposition of aluminium hydroxide at a temperature of 973-1123 K(700 °C-850 °C).

Another paper$\ce{^{[2]}}$ shows aluminium hydroxide being heated from room temperature to 1200 K(926.85 °C) where the exact temperature of decomposition products i.e $\ce{Al2O3}$ is noted:


  [...] we find the first and the smallest endothermic peak at 519
  K, which is due to the partial dehydroxylation of gibbsite
  ($\ce{Al(OH)3}$) and formation of boehmite ($\ce{AlOOH}$). Endothermic
  peak at 585 K corresponds to two processes: (i) transformation of
  gibbsite to phase $\ce{χ-Al2O3}$ and (ii) additional conversion of
  gibbsite to boehmite. The first process is in accordance with the
  results obtained. Another endothermic peak at 815 K is due to
  decomposition of boehmite and formation of alumina $\ce{γ-Al2O3}$. The
  obtained TG curves show clearly three steps of weight loss. The weight
  loss in the first step (about 5 wt.%) is due to the partial
  transformation of gibbsite to boehmite; the second step (about 25
  wt.%) corresponds to decomposition of gibbsite to boehmite, otherwise
  to $\ce{χ-Al2O3}$. The last step of around 3 wt.% relates to the
  formation of $\ce{γ-Al2O3}$. Further observations show that the total weight
  loss is equal to 33 wt.%. All these transformations of gibbsite and
  the chemical composition are confirmed by the XRD analysis.


References


Chemical kinetics and reaction mechanism of thermal decomposition of aluminum hydroxide and magnesium hydroxide at high temperature (973-1123 K) by Ienwhei Chen, Shuh Kwei Hwang, and Shyan Chen, Ind. Eng. Chem. Res. 1989, 28, 6, 738–742 DOI: https://doi.org/10.1021/ie00090a015
http://przyrbwn.icm.edu.pl/APP/PDF/131/a131z3p62.pdf
Bhattacharya, Indra &amp; Das, S. &amp; Mukherjee, P. &amp; Paul, Subir &amp; Mitra, P.. (2004). Thermal Decomposition of Precipitated Fine Aluminium Trihydroxide. Scandinavian Journal of Metallurgy. 33. 211 - 219. 10.1111/j.1600-0692.2004.00686.x.(DOI) (Here aluminium hydroxide is subjected to dehydration upto a temperature of 1440 °C)