Order by random()

SELECT 
o.id,
o.name, 
o.city,
o.image,
true as isOffer,
false as isEvent,
false as isPlace
FROM offers o
UNION
SELECT
e.id,
e.name as event_name, 
e.city,
e.image,
false as isOffer,
true as isEvent,
false as isPlace
FROM events e
UNION
SELECT 
p.id,
p.name,
p.city,
p.images as image,
false as isOffer,
false as isEvent,
true as isPlace
FROM places p