sklearn - How to create a sequential pipeline
When you want to do sequential transformations, you should use Pipeline.
imp_std = Pipeline(
    steps=[
        ('impute', SimpleImputer(strategy='median')),
        ('scale', StandardScaler()),
    ]
)

ColumnTransformer(
    remainder='passthrough',
    transformers=[
        ('imp_std', imp_std, ['feat_1', 'feat_2']),
        ('std', StandardScaler(), ['feat_3']),
    ]
)

or
imp = ColumnTransformer(
    remainder='passthrough',
    transformers=[
        ('imp', SimpleImputer(strategy='median'), ['feat_1', 'feat_2']),
    ]
)

Pipeline(
    steps=[
        ('imp', imp),
        ('std', StandardScaler()),
    ]
)