Millwall will start their pre-season friendly programme at Concord Rangers on Saturday July 6.
The match at the Essex club’s Thames Road home will be a 1pm kick-off.
Neil Harris’ side then head to Portugal for a six-day training camp. They will face Portuguese and English opposition before flying back to the UK on July 18.
The other two fixtures confirmed are at Gillingham on July 21 (1pm) and Southend United on July 24 (7.30pm).
Millwall are also due to have a game at The Den the weekend before the Championship season starts, but have yet to confirm the opposition.
The Lions faced German club VfLBochum last July, which doubled as Jimmy Abdou’s testimonial.
Millwall took on Spanish outfit Granada before the 2017-18 campaign.