The 20-year-old centre will link up again with his elder brother Steffon, who has been in impressive form for Toulon since moving from London Irish last summer.
Delon Armitage, the out-of-favour England full-back, is expected to follow the same route this summer and join his brothers in the south of France.
All three are familiar with the region after spending time living in Nice.
Guy Armitage represented England Under-20s at last year's Junior World Championship but he would qualify to play for France at the end of his new contract.
"Leaving London Irish after six years has been a really tough decision," said Armitage, who has spent time on loan with London Welsh and London Scottish this season.
"Moving to France for three years will be a huge challenge but one I feel I'm prepared to take on.
"The opportunity to develop further with Bernard Laporte - alongside Jonny Wilkinson, Matt Giteau and Mathieu Bastareaud - is one I felt I couldn't pass up."
Toulon currently have a strong English contingent on their books including Steffon Armitage, Simon Shaw and Wilkinson, with Andrew Sheridan joining next season.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.