The variability study of 6.7 GHz methanol masers has become a useful way to improve our understanding of the physical conditions in high-mass star-forming regions. Based on the single-dish monitoring using the Irbene telescopes, we selected three sources with close sky positions. We imaged them using the European very long baseline interferometry (VLBI) Network and searched available data on VLBI archives to follow detailed changes in their structures and single maser spot variability. All three targets show a few groups of maser cloudlets of a typical size of 3.5 mas and the majority of them show linear or arched structures with velocity gradients of order 0.22 km s-1 mas-1. The cloudlets and overall source morphologies are remarkably stable on time-scales of 7-15 yr, supporting a scenario of variability due to changes in the maser pumping rate.

Published in the Monthly Notices of the Royal Astronomical Society, Volume 524, Issue 1, pp.599-618