The group traveled to San Diego, California, to pay special visit to Ryan Wilcox, an 18-year-old, who has been diagnosed with leukemia for the second time, reported People magazine.
Platrow, 43, noticed Wilcox post about his story and desire to meet the stars on social media using the hashtag "#ryanstrong," and brought the stars together.
"Let's do this," the actress captioned a photo of Evans, 34, and Downey Jr, 51, at an airport tarmac on their way to San Diego.
"Today @ryanwilcox0303 got a little surprise. Thank you to the incredible #chrisevans and my better work half @robertdowneyjr for being the men you both are. And thank you to the Wilcox family for your hospitality!" she wrote in the caption.
The trio spent the afternoon hanging out with Wilcox and his family and signed a few autographs - including one on his bedroom wall.