The battle was in the valley. Three men climbed above it.
וַיַּעַשׂ יְהוֹשֻׁעַ כַּאֲשֶׁר אָמַר לוֹ מֹשֶׁה לְהִלָּחֵם בַּעֲמָלֵק וּמֹשֶׁה אַהֲרֹן וְחוּר עָלוּ רֹאשׁ הַגִּבְעָה׃
שמות יז:י
"And Yehoshua did as Moshe told him, to fight with Amalek. And Moshe, Aharon and Chur went up to the top of the hill."
Shemos 17:10
When his hand was up, Israel prevailed. When it came down, Amalek did.
וְהָיָה כַּאֲשֶׁר יָרִים מֹשֶׁה יָדוֹ וְגָבַר יִשְׂרָאֵל וְכַאֲשֶׁר יָנִיחַ יָדוֹ וְגָבַר עֲמָלֵק׃
שמות יז:יא
"And it was, when Moshe raised his hand Israel prevailed, and when he let his hand down Amalek prevailed."
Shemos 17:11
So they held his arms up until the sun went down.
וִידֵי מֹשֶׁה כְּבֵדִים וַיִּקְחוּ אֶבֶן וַיָּשִׂימוּ תַחְתָּיו וַיֵּשֶׁב עָלֶיהָ וְאַהֲרֹן וְחוּר תָּמְכוּ בְיָדָיו מִזֶּה אֶחָד וּמִזֶּה אֶחָד וַיְהִי יָדָיו אֱמוּנָה עַד בֹּא הַשָּׁמֶשׁ׃
שמות יז:יב
"And Moshe's hands were heavy, so they took a stone and put it under him and he sat on it. And Aharon and Chur supported his hands, one on this side and one on that, and his hands were steady until the sun set."
Shemos 17:12